To suppress the higher order harmonics in monochromatic light come from a synchrotron radiation source and to improve the polarization characteristics of the source in a long wavelength, this paper analyzes the polarization characteristics of an triple-reflector polarizer Au_Si_Au and discusses its suppress abilities for higher order harmonics based on the concept of triple-reflection polarizer. It compares the characteristics of Au_Si_Au，Au_SiC_Au and Au_Be_Au triple-reflector polarizers for suppression to higher order harmonics and improvement on polarization characteristics. Obtained results show that the triple-reflector polarizer Au_Si_Au can suppress the higher order harmonics more effectively than other polarizers. Furthermore, After the light is reflected by the polarizer, the reflectance of the P component almost can be zero and that of the S component can totally blocked in the photo energy down to 30 eV. Moreover, the polarization of the source has been improved up to 99% in wavelength of 20 eV.