Magnesium phosphate cement (MPC) can be used to solidify heavy metal ions in hazardous waste, and the introduction of heavy metal ions can significantly reduce the long-term mechanical properties of the solidified body. A method for strengthening MPC solidification of Pb²⁺ by adding K-struvite was proposed to solve the problem of the mechanical property loss in the later stage of MPC solidification and ensure that the toxicity leaching meets the requirements. The results show that the leaching concentrations of K-struvite enhanced the MPC solidification of Pb²⁺ at 28 d and 180 d are 0.63 mg/L and 1.91 mg/L, the compressive strength at 180 d is similar to that of MPC uncured Pb²⁺ (without K-struvite), thus improving the mechanical properties of MPC and meeting the toxicity leaching requirements. The strengthening/solidification effect of K-struvite is mainly due to the improvement of the compactness and adsorption of the solidified body, improving the long-term solidification effect on Pb²⁺.