We collected 21 soil samples from different foresr soil in Chang Bai Mountain and built a mathematical model on infiltration coefficient of forest soil under saturation with theory(1) of quantification. Results are following.1.The mathematical model about infilfration coefficient of forest soil under satyration is drawn;2.First factor, which affects the infiltration is the interaction between soil texture and soil unit weight.According to the importance of other factors, we hace soil texture, unit weight, root quantity in soil and interaction retween the last two;3.Theory(1) of quantification could be used easily with higher pricision for estimating infiltration ofefficient of froest soil under saturation.