This work presents a workspace solving approach for a tomato harvesting manipulator with seven degrees of freedom (7 DOF) based on a special envelope method. In this method, the joints of the manipulator was categoried into two groups, and the workspace boundary of each group was formulated according to the envelope equation by MATLAB and AutoCAD by considering the joint limits. To analyze the reachable and dexterous workspace, the position and orientation of the last link was plotted. The results show that the width and height of the wrist workspace of the tomato harvesting manipulator are 1.5 and 1.2m respectively, which can satisfy the tomato picking requirements. The reachable workspace and the dexterous workspace are affected by the length of the last link and the end-effector. In order to minimize the hollow in the workspace, it is necessary to choose the rational length of the 5th link and the 3rd link and the angle range of the 5th joint. The algorithm can make the workspace visualized and be applicable to all practical manipulator configurations.