about the coax balun 's vswr

You might want to elaborate a little more. A balun, when properly designed, can perform far better than that.

it's the most simple coax balun, input port's skin is grounding, output port connect two 25Ohm Respectively.

---------- Post added at 04:42 ---------- Previous post was at 04:40 ----------

the length is lambada/4,

What's the system impedance? How are the 25 ohm resistors connected, series or parallel? What are they for?

The System impedance is 50 Ohm, two 25ohm is series connected to output port. Is that right, or should be two 50 ohm
series connect. but why? i think when two 25ohm is setted, the circult can be matched.

Yes, that's correct.

What are you driving the balun with, and how are you measuring the v.s.w.r. ?

i just simu it in ADS(agilent), S-parameter Simulation. Input 50ohm.output 2*25 ohm series connect.

In which case, I'm sorry I can't help. I'm used to working on real life equipment, not simulations.