Due to the characteristics of single oil source and severe pollution in the power shift steering transmission of tracked vehicles, the shift operating valve system may be easily clamped and can not work sensitively. A contamination radial force model was proposed to explore the most sensitive clearance range of the hydraulic system under the given conditions. A test bench was built to verify the accuracy of the model. The result shows that the radial force reaches the maximum when the average clearance lies between 900~1000 μm, which is substantially sensitive to the given situation of power shift steering transmission. The eccentricity between the valve body and core has a great impact on the performance of the spool valve. The leakage quality increases and the contamination clamp force when the eccentricity increases at an identical clearance. 