Thanks for all replies!
mhagain, according to the specification in WebGL 1.0, the size of default depth buffer is 16 bit integer.:)
I have struggled this problem throught this weekend.due to the limitations in WebGL,Nether Lineard Depth Buffer nor Logarithmic Depth Buffer is appropraite to be applied in my situation.
maybe ,the only way is to adjust the projection plane dynamicly.