The degree of freedom of the 3-CRS/SP parallel mechanism is analyzed by using the screw theory,the distribution of degree of freedom of the mechanism is determined,the result is verified by the modified G-K formula. And the inverse position solution of the parallel mechanism is got through the method of analytic geometry,the length between the three vertexes of the moving platform is regarded as the restrict condition to construct the constrained equations,the positive solution of all positions under the closed solution of the mechanism is studied. Finally,the appropriate motor is added to the mechanism,the boundary points that the intermediate branch chain in different lengths are searched,and the reachable workspace diagram of the mechanism is drawn by the MATLAB software.