I have a opamp with dynamic switched capacitor common mode feedback and want to know how to simulate the DC gain of opamp with dynamic CMFB, if use the Spectre command PAC and PSS ??

I recommend using PAC. If you didn't have the dynamic CMFB you would use AC, presumably using one of the techniques that Kundert talks about in his book (section 3.4.1), such as Middlebrook's method, to avoid the problems that occur when you open the loop. With dynamic CMFB, you would do exactly the same thing, except replace the DC analysis with a PSS analysis and replace the AC analysis with PAC.

-August