After completed download anime series file,i only can hear a sound but no picture.Why?
although is customary to give details of what you downloaded when asking for help, and I don’t usually “guess” at what that might be, but I’m pretty sure you need the proper codecs to view the video.
Here are some links to codec packs…
DO NOT install both…
CCCP (best for playing of files)
k lite codecs (best for playing, editing and encoding)
http://www.free-codecs.com/download/K_Lite_Codec_Pack.htm
if you select the k lite pack, don’t just install all the options, or you will surely have problems (default settings work fine for nearly every user)
Suspect
I’m downloading this Zoids Chaotic Century file.After convert it to avi file with Winavi,now there is a picture but no sound.
Your question is related to video encoding.
Since you did the encoding, you have the codecs, so it can’t be a codec issue.
Your problem is you did something wrong during your encoding process. I would try it again, and pay more attention to you winavi program.
If you still cannot get it to work, you should contact winavi support.
Suspect
Maybe you can download a k-lite codec for a try. 
I’m downloading this Zoids Chaotic Century file.After convert it to avi file with Winavi,now there is a picture but no sound.
Was this one big .torrent with all 67 episodes in .mkv files with English audio only and no subs?
If so all you need is CCCP to play the files.