In order to realize an optimal balance between the efficiency and reliability requirements of road models, a road modeling method for digital maps based on cardinal spline is studied. First, the cardinal spline is chosen to establish an initial road model, which is specified by a series of control points and tension parameters. Then, in view of the initial road model, a gradual optimization algorithm, which can determine the reasonable control points and optimal tension parameters according to the degree of the change of road curvature, is proposed to determine the final road model. Finally, the proposed road modeling method is verified and evaluated through experiments, and it is compared with the conventional method for digital maps based on the B-spline. The results show that the proposed method can realize a near-optimal balance between the efficiency and reliability requirements. Compared with the conventional method based on the B-spline, this method occupies less data storage and achieves higher accuracy.