Hello HouJun,
Your model looks fine.
You said there is a change from stripline to mictrostrip line, but I can't see it in model.
The problem could be when trying to match the coupler from striplinr to microstrip.
In general, the striplines are between two conducted ground and microstrip is a conductor above ground.
How did you bypassed the ground planes for the microstrip transmission line?
I think you grounded the main signal transmission by this.
Please check it.
IH