why was this film banned in China?

because the villages were forced to hold the prisoners by the communist army. In that time, three major powers were in China: Japan, the republicans of China and the communist of China. The innocent villages were drag into these mess(war) by the soldiers of the the communist army and therefore it (the communist party) is responsible for the eventually masscre-->

these is the ultimate truth behind every war: no matter for what causes, countries are fighting against each other, in most cases innocent citizens who don't care about the causes will find the death!!!!

But you know, that there is a unwritten rule in the society of modern China, the communist can't be blame for anything...(especially using citizens to win the WOII) so that is why it is banned.

I'm Chinese.
In China, people say the reason is that the film does not show any good things done by the CPC, and evil things done by the Japanese intruders.
The film regulators are too conservative to release such movies.

However, we are free to buy the film DVD anywhere ;)

IIRC the PRC government was pissed off because Jiang entered the Cannes Film Festival without their consent, which is a ridiculous reason to ban the movie.

I think the movie is banned in China not because the content,but the Chinese audiences. Most Chinese people have a common anti-Japanese feeling,I think they couldn't handle the fact that the Jap murderers (especially the jap prisoner)not ony get away with it,but also execute Ma da san in the end.
BTW,I can't figure out why did Jiang wen want his movie to end like this,apparently letting a POW execute another prisoner is not quite possible.

To add to what has already been discussed...I was told the reason why the movie was banned in China was because it did not portray the Japanese military as being bad enough. This explanation was from one of my ESL students here in northeastern China, who has seen the film. This explanation makes sense to me as there is deep seated resentment towards Japan stemming from WW2. Additionally, most of the movies made in China dealing with that period of time either portray the Japanese as being incredibly evil (henceforth setting up the Chinese soldiers as being super-heroic) or the Japanese to be complete idiots. While I personally considered Wen Jiang's screenplay to be very condemning of the Japanese, obviously the Chinese government did not agree.

Reason 1. The Chinese villagers were portrayed very naivee and even stupid, no sense of resistance until when they had no choice. This is very different from early anti-Japanese movies.
Reason 2. It could stimulate the Chinese nationalism within China. The movie reveals the very evil side of the Japanese army in the end of the movie together with the powerlessness of Chinese people. Too striking that it could stimulate the existing Chinese-Japan conflicts. As the director and actor Jiang Wen said he wants to make a movie really shows the truth of anti-Japanese war and he did it. Right now even though China and Japane still have conflicts, too much nationalism toward Japanese will do no good to Chinese government.

There is indeed nothing controversial in China about anti Japanese sentiment. In fact it is still fostered to some extent by the Party. To show too much sympathy for the Japanese, even in this day and age, could be construed as unpatriotic.

As opposed to a shallow blockbuster like "Nanjing! Nanjing!" (which merely shows brave Chinese soldiers heroically fighting an impossibly evil Japanese oppressor, and where the only 'good' Jap commits suicide) however, this film makes a mockery of Chinese resistance and its immoral message would be considered dubious as best by Communist ideological standards.

https://chinacopyrightandmedia.wordpress.com/2012/10/17/devils-on-the-doorstep-an-interesting-look-into-film-censorship/

Here's the answer in extreme details.I feel pretty bad for Chinese film makers and directors after reading it.It's almost impossible to make any good movies under such censorship.