The deformation force is an important factor affecting the forming accuracy of parts in the incremental forming process of sheet metal. This paper proposes an analytical calculation method of the deformation force based on pure shear deformation. After assuming and simplifying the factors affecting the deformation force, a graphical method is used to approximate the contact area between the forming tool and the sheet metal. A forming test is also designed. In addition, the deformation force is measured in the experiment, and its theoretical analysis value is compared with the actual measurement value of the forming test to validate the analytical method of deformation force calculation. The results show that the radial forming deviations are 28.5% and 22.5%, the axial deformation force deviations are 9.8% and 16.1%, and the forming force deviations are 6.3% and 10.3%, which demonstrates the effectiveness of using the analytical method to calculate the deformation force.