In order to improve the adaptation of quadruped robot for various terrains, the major problem is how to build up the smooth transition algorithm from trotting gait suitable for flat terrain to walking gait suitable for rugged terrain. Firstly, one gait cycle of trotting gait and walking gait are analyzed for finding out all the potential switch points and access points. Secondly, based on analysis of two cases that can occur when robot change its gait from trot to walk, the optimal transition algorithms, which can let gait transition most smooth and waste least time when robot keep stability are proposed. Finally, the model of quadruped robot is built based on the robot simulator Webots, and the simulation results show the validity and feasibility of the proposed gait transition method.