The effects of unitary retarder system and binary complex retarder system on setting time, fluidity and mechanical properties of high strength magnesium phosphate repair mortar were studied. The results show that in unitary retarder system, compared with borax and sodium tripolyphosphate, the retardation effect of sucrose alone is the best. But excessive content of sucrose significantly reduces the fluidity and early mechanical properties of magnesium phosphate repair mortar. In the binary complex retarder system, the retardation effect of borax and sucrose is better than borax and sodium tripolyphosphate. Under 5% (mass fraction) borax and 2.5% (mass fraction) sucrose, the setting time of magnesium phosphate repair mortar reaches 18 min, the fluidity is 230 mm, and the strength of 1.5 h, 3 d and 28 d is 22.6 MPa, 56.8 MPa and 62.4 MPa, respectively. The setting time and early strength of magnesium phosphate repair mortar achieve dynamic balance.