I found a strange thing,

IPoint3 vertObjectSpace = mesh.GetVert(vertID);

This position is not in the world space.This is totally different from mxs.So we have to deal with it in the following way:

IMatrix3 tmObj2World= i.GetObjectTM(0, iTimeRange); // where i is your IINode

IPoint3 vertWorldSpace = tmObj2World.PointTransform(vertObjectSpace);

If we subtract these two values.The above method is really not simple, and there are accuracy problems.Ask the experts if there is an easier and more formal way.