The contamination lock failure of the hydraulic spool valve caused by oil contamination is one of the common faults of armored tracked vehicle. The contaminant wear particles enter the clearance between the sliding parts with the hydraulic oil and then obliteration occurs. Then a filter cake that impedes the motion of the spool is formed. The forming mechanism of the filter cake was analyzed, and the models of the cakes forming process and the contaminant lock force were built based on the theory of cake filtration with the introduction of particle distribution influencing factor. The model was verified by an experiment conducted on the designed experimental platform. The model could be used to predict the contaminant lock force of spool valves and guide the design of spool valves and hydraulic systems.