A simple site test method of time constant Td 0 ′ for generators with self-shunt excitation system is studied. Through cutting off the source of trigger pulses for silicon-controlled rectifier（SCR）， the discharge time constant of circuit consisting of the rotor winding， two conductive SCRs and the secondary side of excitation transformer， can be measured. This method shortens test duration， moreover the error is smaller than that of usual method. Rationality of the method is proved in theory of（linear）circuit superposition principle.And error analysis based on practical situation， indicates that the error of this method can satisfy engineering requirement. Finally， a site test example is given. The method can be applied to the units with self-shunt excitation system only if its automatic voltage regulator（AVR） is possible to cut off the source of trigger pulse for SCR.