Thanks for your reply.
It is because the defferent speed of every object.
for example:
due to the different speed, the distances between the leading object and other objects are changing per 10 ms, and my camera is always looking at the leading object, so from this camera's view, the other objects are shaking, for example, the first 10 ms, object 2 is behind 3 pix to the leading one, after 10 ms, object 2 is behind 5 pix to the leading, and maybe after another 10ms, object 2 is behind 1 pix to the leading. I make the camera focus on the leading one, so it seems that the object 2 keep on shaking.
But I don't figure out how to solve this.