During the processing of the servo valve spool machining, the overlap of valve spool's each edge should take zero grinding. The grinding processes including spool overlap test and grinding. In this paper, the authors used a new test method, which simulates working condition of servo valve. Hydraulic cylinder is used as the load of servo valve in this test method, and the hydraulic cylinder is in no-load operation. This test method realized the automatic switch of test valve, and it determined the amount of grinding through complete measuring four edges overlap values, furthermore, it realized measuring the total amount of overlap, thus reflect the flow characteristics of a valve. This is a new automatic method for measuring servo valve overlap.