In order to unite the merits of ICCD (intensified charge coupled device) and EMCCD (electron multiplying charge coupled device) for detection of the weak signal in the high resolution Thomson scattering system from strong radiation background, a second generation image intensifier, lens coupled with an EMCCD are used together as a detector. The signal photon flux is so low in the actual measurement situation that the gain of the I.I., on-chip multiplication gain of EMCCD and on-chip binning scheme might all need to be utilized to enhance the detection capability or to set lower demands for other devices. At the same time, however, these amplification processes bring unexpected noise in addition to the detector noise itself, which will further degrade the signal to noise ratio (SNR). This paper will focus on three points. Firstly, the three gain methods, including MCP gain, EM gain and binning are theoretically described. Secondly, the amount of increase in signal counts based on this detector combination is experimentally investigated at various gain settings, as well as the total noise. Finally, a gain selection disciplines aiming to obtain an optimum SNR is generalized according to the comparison between test results and theory.