Aimed at the problem of mapping of multijoint robot workspace to parameter space in non-structural environment, a C-space obstacle calculation method based on improved pseudo distance method is proposed. First, traditional method of pseudodistance was improved. The corresponding C-space obstacle was obtained with the consideration of geometrical dimensions of mechanical arm. Then, the C-space of sloped obstacles was solved by using translation and rotating homogeneous coordinate transformation. Finally, validity of space model of three-joint robot was verified by Matlab simulation. The results of the study provide the basis for the programming of the robot's real time obstacle avoidance path.