Abstract: The structure of EMCCD is introduced. The functions of signal amplification and noise restraint were realized by inserting the all-solid-state electron multiplying structure in the EMCCD chip and amplifying the charge coming out in horizontal transfer before detection according to the noise resources of EMCCD. Through a theoretical analysis of mathematical models for the main noises of EMCCD, three characteristics of EMCCD noise were-summed up:the EM gain can effectively restrain the read out noise; the noise factor generated during the EM gain process has the function of amplifying the noise produced before the multiplying structure; the effect of CIC on the EMCCD performance is serious. It is pointed out that the improvement of EM gain, aggravation of cooling and optimization of clock wave form can effectively restrain the noise.