According to the different operating environment and working conditions, the collision can be divided into unconstrained collision and constrained collision, including immovablepoint collision and movablepoint collision. Firstly, the impact dynamics model of the serial robot system was established based on Lagrange equation, and the solving model of external impulse was derived from classical collision theory and recovery coefficient equation. Secondly, with respect to unconstrained collision, the concept of impact motion mapping matrix was proposed on the basis of impact dynamics analysis, and the evaluation index named impact motion performance was constructed for evaluating the ability to maintain motion stability under external impulse action. Finally, the ideas and methods for solving problems of immovablepoint collision and movable-point collision in kinematically redundant serial robot were transformed into optimizing robot’s pre-collision posture by choosing the minimum external impulse generated during the collision as the objective for optimization. Taking the planar threebar robot as an example, the impact motion analysis and optimization design of unconstrained collision and constrained collision were carried out respectively. The results showed that the external impulse would rise significantly when robot was in or near singularity, and its structural parameters had a great effect on the impact motion performance. The external impulse generated from collision can be reduced effectively by means of pre-collision posture optimization, which was very useful for improving the stability and security of the system.