The multipactor effect has to be dealt with seriously when designing and manufacturing high power microwave devices used in space, as it will cause inreversible damage to devices and hence the whole system fails to work. Lowering the secondary electron yield of device surface is an effective way to suppress multipactor effect, which can be realized by creating trapping structure or depositing nonmetallic materials with low secondary electron yield on the surface. However, these treatments will result in electrical performance changing even to an unacceptable extent in some cases. To solve this problem, the deposited materials with conductivity as good as metals' should be used, besides, they should be chemically inactive in air. Taking the above into account, the method of suppressing the secondary electron yield of silver plated surface of device by magnetron sputtering platinum is proposed and investigated in the present paper. Firstly, platinum film with a thickness of 100 nm is deposited on silver plated aluminum alloy substrates by magnetron sputtering, and secondary electron yields of substrates with and without deposited platinum film are measured with the bias current method. The experimental results indicate that the maximum value of secondary electron yield and the first cross energy of silver plated aluminum alloy sample are 2.40 and 30 eV, respectively. After depositing platinum film on sample, these values change to 1.77 and 70 eV, a reduction of 26% in maximum of secondary electron yield is achieved. Secondly, universal law, Vaughan model, Furman model and Scholtz model are used to fit the experimental data of secondary electron yield, and the results indicate that only Vaughan model accords well with the property of secondary electron yield in an energy range of 40-1500 eV, and corresponding parameters are also obtained. The Chung-Everhart model is used to fit the secondary electron spectrum curve, and the fitted work function is 9.9 eV. Finally, the simulation of multipactor threshold of Ku-band impedance transformer is carried out by using a software with utilizing the experimental data and fitted results of secondary electron emission of samples. The simulation results indicate that the multipactor thresholds by utilizing the data of samples with and without platinum are 7500 W and 36000 W, respectively, which means that the large increase of multipactor threshold of high power microwave device can be achieved by depositing platinum film on the surface. The method proposed and results obtained in the present work provide a reference not only for studying the secondary electron emission of metal, but also for suppressing the multipactor effect of high power microwave device.