The invention provides a design method for the rigidity of a spiral spring of a car. The offset frequency of a suspension is determined according to driving smoothness, the rigidity of the suspension on one side is calculated, and then the rigidity Cs of the spiral spring not considering the contribution of a transverse stabilization rod is worked out according to the characteristics of a suspension guiding mechanism. The design method further comprises the following steps that firstly, a finite element model of lower swinging arms and the transverse stabilization rod of the suspension is built, the transverse stabilization rod is limited, vertically-upward action force F is applied to a spherical hinge of each swinging arm, the vertical displacement of the center of each spherical hinge is calculated through a finite element calculation tool, and an equivalent vertical spring rigidity Cb at the position is obtained; secondly, the equivalent vertical spring rigidity Cb is converted into a suspension spring rigidity component Csb; thirdly, the rigidity Cst of the spiral spring affected by the rigidity of the transverse stabilization rod is calculated. According to the design method for the rigidity of the spiral spring of the car, the influence on the rigidity of the spiral spring of a suspension system from the transverse stabilization rod is taken into full account, and an accurate value is obtained through calculation.