Considering the simply-supported cylindrical shells， the modal participation in dynamic responses is studied by consider? ing the influence of sine and cosine modes， and a method for determining the order of modal truncation according to the distribution characteristics of modal participation factors is proposed. The dynamic responses of cylindrical shells under the impact excitation and rotating traveling wave excitation are obtained by the superposition of the sine and cosine modes， and the reliability of the deter? mination method is verified by the convergence of responses. The results of theoretical calculations and finite element simulations show that the influence of sine and cosine modes must be considered simultaneously in calculation of the dynamic responses for cy? lindrical shells， which is different from the case of modal characteristics analysis. When cylindrical shells are subjected to impact ex? citation， the participation degree of each order sine and cosine modes is related to the location of the excitation point and observa? tion point. When cylindrical shells are subjected to rotating traveling wave excitation， the participation degree of each order sine and cosine modes is closely related to the order and frequency of the excitation.