Thank you for your reply! I did the following work according to you instructions

1. First I check the mode profile of mode source as shown below, FDTD and mode source field are set to be 2 micrometers larger than the waveguide so I think theres no truncation?

2. I set a power monitor at the end of propagation to see T, basically its the same as result from time monitor.

3. I set the "frequency dependent profile" in mode source and tried 10 to 20 frequency points, but the loss is still there as before.

4. I also set a movie monitor on XOY plane to check the pulse shape in propagation, but in the video it looks like a plane wave and tranmitt to both directions???

The results are really confsing, what else can I do? Looking forward to your reply!