The invention discloses a three-phase output current amplitude limiting method for a static reactive synchronous compensating device for a power grid. The three-phase output current amplitude limiting method comprises the following steps: sampling three-phase load current filtered by hardware and instantaneous value of voltage on line side, and calculating to obtain a three-phase order output current value; comparing an order current maximum value of the last period through a software to calculate a corresponding proportional coefficient to restrain output order current of the next period. According to the three-phase output current amplitude limiting method, the proportional coefficient is introduced, the order output current amplitude limiting mode is changed into a scaling-down mode; if the output current exceeds device capacity, output according to the maximal value, and the output waveforms are not affected, so that the compensation effects of DSTATCOM are improved. The defects of the conventional output amplitude value retraining method are overcome. The method is simple and practical, strong in interference resistance and strong in engineering generalization performance, so that over-capacity output of the static reactive synchronous compensating device for the power grid is avoided, the compensating effects are improved, and the interference resistance of the device is strengthened.