Space solar power station is a kind of space structure with large size and high flexibility. It is far larger than the previous spacecraft in size, which results in new phenomena and new problems on the study of dynamical characteristics. The kilometerscale dumbbell model was studied. The Hamilton’s dynamical model on orbit was established under the effect of gravity gradient. The symplectic Runge-Kutta method was used with different combinations of parametrical values to obtain dynamical responses. By comparing the simulation results, the quantitative relationships were determined respectively between the size of space structure and the effect of gravity gradient on orbital motion and attitude motion. It is found that: due to the gravity gradient, the coupling phenomenon between attitude motion and elastic vibration occurs; the attitude motion has great influence on the external envelope curve of elastic vibration response, while the period of it is changed by elastic vibration.