The rapid setting characteristic of potassium magnesium phosphate cement (MKPC) limits its application and development in engineering fields. Effectively prolonging the setting time is one of the key technologies for its engineering application. In this study, the effect of borax/triethanolamine composited retarder on the setting time, compressive strength, phase composition, micromorphology, pore structure and hydration heat release properties of MKPC was studied, and the retarding mechanism was discussed. The results show that the setting time of 26~100 min can be controlled by using the composited retarder under the condition that the 7 d compressive strength is greater than 20 MPa. Triethanolamine molecule is adsorbed on the surface of MgO and plays a role in water blocking, which significantly reduces the standard hydration heat release rate and standard hydration heat release, and achieves the retarding effect. The decrease of K-struvite content and the increase of pore (larger than 10 nm) volume are the main reasons for the weakening of compressive strength.