About match network from PA to switch ?

Compute the PA's output impedance, use a low pass passive network and match it to 50 Ohms using Smith Chart.

Rgds

Usually the RF switches have good return loss (better than –20dB) and do not require a match (if the output of the PA is 50 ohms).
Do a separate evaluation of the switch.
You have to verify the return loss of the switch for all possible cases (unused ports loaded, or unloaded).

if it's RL is bad,
you can get S-para from the manufaturer.

I design the mobile .At the present ,I find the harmonic is very poor ,I think maybe related to the
match network from PA to Switch .Do you agree with me ?Best regards !

What kind of switch do use? If you have used MESFET and also if you drive the switch very high power, the switch will produce harmonics due to forcing the mesfets into nonlinear region. This is valid of course with PIN diode switches.

Check the absolute maximum power range of the switches and put at least %25 margin.

Second, if the PA has an optimum impedance to deliver the maximum power at the frequency of interest, in this case you should match OptImp-->50 with an appropriate circuit.

Yes ,I know . But how to test the load impedace of PA .Because I must know the impedance then match to the 50 ohm s .Right ? Best regards !

To find the load impedance of the PA you need a Tuner or a Load Pull test-set.
The latest PA’s for mobiles that are on the market now, have internal output match to 50 ohms, and do not require any external match.
What PA are you using? Let me know this, and I will tell you if you need a LPF at the output or not.

We use the skywords PA 77304? Do you familiar with it ?

I know very well CX77304.
According to Skyworks datasheet this PA have a second harmonic level of –7dBm @ full power in GSM900 (even more for DCS1800).
I can say that I measured, in average, –9dBm 2nd harmonic for this PA.
From a system point of view to meet the GSM spec for harmonics (–30dBm @ antenna connector), this PA requires at a LPF (~15dB rejection) and a diplexer, if is a dual-band design (~15dB rejection). You need some margin also.
The LPF and the Diplexer could be in any format (coupler+LPF, switchplexer, etc)
This PA have internal 50 ohms output match and do not require an external match.