The effect of plasma on the microwave propagation properties is investigated experimentally by using a high power microwave combiner. The plasma is generated by radio-frequency breakdown on the coupling slit of the combiner through changing the power or pulse width of the prime microwave source. The plasma diffuses from the slit to the high power microwave transmission channel, and induces the absorption of microwave energy and the rotation of microwave polarization. The results show that the spread velocity of plasma is about 1 μs/cm, and the duration is about 5 μs. The polarization rotating angle is determined by electron density and microwave frequency. The maximum rotating angle is 4.1° while the number of electron is about 3.7×1015.