House Bank" is a new mode of house tenancy, in which the rent income in free-rent period is the primary revenue source. The rent income in free-rent period mainly depends on values of house rent and free-rent period and leasing house quantity. In this paper, the rule of in-and-out house quantity variation in a given period is analyzed. It demonstrates that the probability of in-and out quantity of house in a period meets the four characters of Poisson distribution and consequently this probability meets the Poisson distribution. Based on the characters of Poisson distribution, in this paper, the rent revenue of "House Bank" is analyzed and the way to find reasonable free-rent period is discussed.