Last time I watched the movie, they all speak black english, actually I really didn't know what they meant and couldn't catch their accent, words and the conversation speed. I wonder how I can learn that kind of English, even I can't speak this style of English, I also want to understand their conversation easily, please advise how I should do that. Thanks.
I take it you are watching American films set in urban surroundings, usually in the black communities. The structure and sounds are the results of two centuries of native language pronunciations and structure being adapted to English - or perhaps rather that English forms have been adapted to fit into these forms.
I would not advise anyone learning English to set out to mimic this. It would seem very odd to anyone with whom they have to communicate [apart from these groups, who do not always like others mimicking them]. There are sites on the net in which you can find discussions of idiom and pronunciation of various black dialects.