Rocket cabin shells are usually constrained by clasp frocks in ground tests， which will have a significant influence on structural vibration characteristics. A model of coupled cylindrical shells with elastic boundary constraints is established in this paper. The normal constraint stiffness was obtained by the contact calculation model between clasp frocks and cabin shells， and the supplementary functions are used for expressing the bending vibration displacement of the cylindrical shells. Results showed that measured data follow the same trends as the theory model prediction； the low-frequency vibration response of cabin shells is deeply suppressed by frock boundary constraints， indicating great influences from both constraint stiffness and relative position.