The impeller is the important pneumatic part of centrifugal fan, and its structure performances are key factors which affect the whole performances of fan. The CAD module of centrifugal fan can realize the automation of aerodynamic force calculation. According to demands, computer can automatically complete aerodynamic force calculation and further determine major geometric parameters of impeller of fan. Speed coefficient and diametral quotient are two important parameters reflecting the character of ventilating fan. The relation curve between the speed coefficient and diametral quotient of various fans is plotted in this paper. The CAD module of impeller of centrifugal fan can realize such functions as aerodynamic design and parameterization drawing of impeller, and can accomplish rapid response from receiving design parameters to profiled impeller of fan, so that it can improve the quality of drawing.