Thank you very much for your replies.
Yes, I combine turret and LOS. The built-in LOS and turret behavior are great. But where the cone / LOS starts from (from image points?), I mean, if I want to overlay the LOS cone with sprite, how does the radius expansion work? .I tried to make my own sprite ("movieclip") of LOS ,but the result is not good enough.
Anyway ,thanks for your replies.