As JonLeung recommended the "PNGGauntlet" that can shrink png's file size, I tried it. But the result was not so inspiring. I happened to find another PNG-optimizing freeware named "advancecomp". Its homepage is
http://advancemame.sourceforge.net It can compress PNG smaller and cost less time than PNGGauntlet. Here are some comparisons between them (Both software use its max compressing level).
canvas size color type size compressed rate time tool
16128*8064 PNG-24 4065KB 3191KB 78% 26min advancecomp
15456*10304 PNG-24 18740KB 16530KB 88% 32min advancecomp
4608*4352 PNG-24 1874KB 1261KB 67% 4min advancecomp
1376*3360 PNG-8 116KB 103KB 89% 4s advancecomp
4608*4352 PNG-24 1874KB 1699KB 91% 170min PNGGauntlet
1376*3360 PNG-8 116KB 104KB 90% 4min PNGGauntlet
The shortage of advancecomp is that it only works in DOS mode.