The invention relates to an electro-hydraulic servo valve overlap value measuring device and a measuring method thereof. According to the device, stable gas pressure is provided for an oil incoming cavity or an oil return cavity of an electro-hydraulic servo valve; a valve core is driven to move slowly and infinitesimally; gas flow and valve core displacement data are acquired in a valve core movement process; and the overlap value of the working edges of the electro-hydraulic servo valve can be calculated further. The device mainly comprises a motorized translation stage, a contact type displacement sensor, flow controllers, a gas distribution seat, pneumatic sliding stages, gas claws, a gas path system and the like; the motorized translation stage drives the valve core to move slowly and infinitesimally; the pneumatic sliding stages realize the pressing of a processing casing, the clamping of the valve core, and the contact of the displacement sensor and the gas claws; and the flow controllers measure and control the gas flow. With the measuring device and measuring method thereof of the invention adopted, a flow change condition in the work of the electro-hydraulic servo valve can be simulated. The electro-hydraulic servo valve overlap value measuring device has the advantages of high measuring accuracy, high repeatability, high automation degree and easy and convenient operation.