The cubic polynomial interpolation with midpoints is applied to the path planning of robot leg in the complex tunnel environment. A new spider biomimetic robot with 18 joints and 6 legs is designed,which can be flexible movement in tunnel space. The kinematics model of the robot leg is built with D-H modeling method,and the inverse kinematics equation is deduced. Based on Monte-Carlo method,the working space of the foot end is analyzed,and based on the cubic polynomial interpolation algorithm with midpoints for path planning of obstacle avoidance is simulated. Simulation results show that the application of the cubic polynomial interpolation method with midpoints in path planning on spider biomimetic robot leg could guarantee the smoothness of foot movements,avoid the impact caused by mutations of moment,and meet the requirements of obstacle avoidance.