Abstract: Optical transmission is a key issue of the Thomson scattering system due to the weak scattered signal. In this paper, the methods of reducing the stray light and increasing the optical transmittance of the system is discussed. Simulation shows that the transmission of the polychromator is nearly proportional to the electron temperature and it reaches 10% when electron temperature is 1 keV. Scattered signal is very small and there are only hundreds of photons in each channel when electron density is 1×1019/m3. When the electron temperature is more than 300 eV, the measuring uncertainty of the system from photon fluctuation is less than 3%.