Based on the harmonic response analysis and the define of plane stress circle, a new method about the spatial non-uniformity analysis of the moment output for the car wheel dynamic bending fatigue testing machine is provided. The forces applied on the testing machine are analyzed and computed. Then, the forces are applied into the finite element model. The stress dates of the assigned positions are obtained and the curve of plane stress circle is plot. The simulation date and the measured results were compared by the plane stress circle curve graph and parameters. The analysis showed the three main influence forces of the testing machine: the effect of the centrifugal force is dominant for the performance of the testing machine; the gyroscopic moment comes from the coupling between the rotation of rotor and its lateral swinging; and the nonlinear oil film forces are caused by the lubricating oil and the unbalanced rotor rotation. Compared the plane stress circles between the measured results and the simulations when all the three kinds of dynamic load were applied on the testing machine, by the graphic and parameters, the data fit well. It proved that this method is appropriate for the analysis and design of the testing machine.