Which is better for Sapporo snow festival ?

I will go to sapporo next year but cannot go during snow festival (Feb.05-11).

Which period is better to see ice and snow sculpture between before snow festival (Jan29-Feb02) or after snow festival (Feb 17-21)????

You should be able to see the snow sculptures under construction if you go few days before the festival. The ice scuptures, however, are not usually built until just a days before the festival, which may be too late for your schedule. Everything is removed almost immediately afterwards, so there probably won't be anything left to see by the 17th. See this report for more info on what its like before the festival starts:

http://www.japan-guide.com/blog/scott/120204_snowfestival.html

There is also the Otaru Snow and Light festival which runs through the 17th. They don't really have big snow sculptures like the Sapporo Yuki Matsuri, but they do have lots of art installations made of snow and ice.

http://www.japan-guide.com/e/e6706.html