The invention provides a method for testing the response time constant of a servomotor of a mechanical hydraulic servo system. The method comprises (1) changing the limit to the maximum rotation speed of a step motor; (2) a speed controller executing automatic control, increasing the system given value stepwise to make the step motor rotate to the open direction at the maximum speed, reading the displacement of a distributing valve in the open direction by a dial indicator, and recording the relative stroke increase process of the servomotor by a data acquisition instrument and a displacement sensor; (3) reducing the system given value stepwise to make the step motor rotate to the close direction at the maximum speed, and recording the displacement of the distributing valve in the close direction as well as the relative stroke decrease process of the servomotor; (4) repeating the above steps to obtain three groups of data of the displacement of the distributing valve and the relative stroke change of the servomotor when the step motor rotates to the open and close directions at different maximum speeds; (5) calculating the relative displacement of the distributing valve and the relative moving speed of the servomotor according to the acquired data; and (6) calculating the response time constant Ty of the servomotor. The method provided by the invention is less in workload, easy and simple in realization, and accurate and reliable in results, and has no influences on the unit operation security.