Non-circular synchronous belt could be used in the non-uniform speed transmission with large center distance. The closed figure extracted from the synchronous belt wound around the non-circular synchronous belt pulley is composed of curves and straight lines alternately connected. Due to the differences between the closed figure and the pitch curves of the non-circular synchronous belt, the synchronous belt is not always tangent with synchronous belt pulley during the transmission, which will influence the accuracy of the transmission. Consider that, a mathematical model was proposed to calculate this phenomenon above which is called “polygon-like effect” in this paper. By calculating and analyzing some samples on the basis of this model, three key factors that influence the nonuniform transmission ratio of non-circular belt pulley in the transmission were concluded, which include the initial position, tooth number and nonuniform transmission parameter. The more teeth of the noncircular belt pulley have and the smaller the non-uniform transmission parameter is, the smoother the real transmission ratio curve is. And the real transmission ratio curve of master-slave wheel in phase is smoother than the curve of masterslave wheel out of phase. This study could provide the important reference for the tooth number design and manufacture of non-circular synchronous pulley as well as calculating the initial position.