In the space robot self-assembling process, the assembling control becomes quite complicated and important for the nonlinear characteristics, which are caused by the dynamic coupling between the arm and the base, and the collision between the target and the assembling hole on the base. The dynamic model of the space robot with target is established firstly, then the Hertz model is introduced as the collision model between the docking link and the capturing cone, and the operational space control is applied to achieve the assembling operation control under the Cartesian space. Through simulating and analyzing the influence of the stiffness coefficient, the soft and hard behaviors during assembling are discovered, which would provide a basis for assembling control and parameter design. The results would have important academic value and engineering significance on space robot achieving on-orbit self-assembling.