The excitement of second electron in electron multiply process is a gain process actually, which obeys certain distribution rules. The gain is a randomized quantity. The characteristics of input-utput of electron multiply system can be described by a noise factor usually. Because the noise factor is connected with input signals, it can't be used to characterize the gain of multiplication. In this paper, one concept of gain variation factor is given to describe the gain characteristics. Based on the analysis to cascading system of electron multiplication, one more general hypothesis about multiplication process statistics is prompted, and the expressions of noise factor and gain variation factor in cascading multiplication system and their relationships are given.