The problem I want to solve is about 3D sound simulation: For example given Dolby AC-3, which applies 5.1 system, I would like to simulate it by two ordinary speakers of my computer. How can this be done, could you also provide some explanation about the reason?
As @hobbes mentioned, what you are proposing is not possible. It is, however, possible to render a 5.1 signal to ambisonic B-Format and thence to Binaural which can be experienced only with headphones, but it is not possible in the context of a pair of speakers. In any rate the binaural experience is not as immersive as true 5.1 and this will not be possible to achieve unless you are working in the context of a Digital Audio Workstation as opposed to simply using DVD/Blu-Ray playback.