I encountered problem for OIP3 and OP1dB with my circuit.

the circuit is just a RF amplifier.After get proper input and output impedance match,i use pss to get OP1dB and pss+pac to get OIP3.the result is  OP1dB is 12dBm,however,OIP3 is only 6dBm. the sweep range for OIP3 is  -40 to -20dBm, and the 3rd linear region is ok.i wonder why the OIP3 is so small.Would someone like to give me some help or suggestion?  Thanks.

I agree with ACWWong, try to confirm your findings with a transient simulation.

Another approach is to check your testbench by replacing your device level model with a behavioral model. With the behavioral models, you can specify the compression point. You should see the IP3 9.6dB above the compression point. (Be sure to account for the 1dB difference between the input and output referred compression points. The output compresses but the input does not.)

Another thought is whether your model is truly dominated by a third order nonlinearity. The "10dB" difference between IP3 and compression only applies to a third order nonlinearity.