The gain principle of EMCCD has been introduced and Total Signal-to-noise Ratio (SNR) formula has been induced based on mechanism of EMCCD noise. The sensitive detection capabilities of EMCCD which can be strongly influenced by readout noise have been weaken by electron multiplication function. It is necessary to minish the multiplication of EMCCD as much as possible to protect electron multiplying structure. CCD60 should be operated at inverted mode to obtain less total noise. This conclusion is a significant direction to EMCCD’s hardware circuits design.