Hydration and hardening properties of magnesium potassium phosphate cement paste(MKPC) containing composite retarders(CR) were investigated by testing and analyzing its setting time, hydration heat，compressive strength，phase component and microstructure， which were compared with those of MKPC paste with borax(NB) added. The results show that there are two heat releasing peaks on hydration temperature curve of MKPC paste with CR, and the setting time of MKPC paste is prolonged, the early hydration rate of MKPC paste is lowered, the highest temperature and total heat release of MKPC paste are decreased. The hardened MKPC paste with CR generates more main hydrates magnesium potassium phosphate hexahydrate(MgKPO4·6H2O MKP), which grows more perfectly and stably with few defects and cracks resulting in more perfect microstructure and high late compressive strength of hardened MKPC paste.