Abstract: Electrolytic manganese residue (EMR) can slow down the hydration rate of polymer magnesium phosphate cement composite mortar, prolong the setting time and improve the microstructure. Through macroscopic physical and mechanical properties, working performance, combined with microscopic means such as XRD, SEM, TG-DTG and NMR techniques were used to investigate the mechanism of the influence of EMR dosage on the early macroscopic and microscopic pore structure properties of magnesium phosphate cement. The results show that the addition of EMR can improve the working performance of the slurry, enhance the later strength and effectively refine the pore structure. The 28 d compressive strength value of adding 2wt%EMR reaches 49.5 MPa, and the strength of 3wt% and 4wt% additives is significantly reduced. In addition to the elongated tree like struvite (MgKPO₄·6H₂O) and block like MgO in the raw material, Mn elements participate in the reaction to form manganese containing compounds, and the hydration products overlap with each other to form a dense microstructure which refines the pores. TG-DTG curve at 100℃ appeared obvious heat-absorption peak corresponds to the heat-absorption dehydration phenomenon of guano stone, the mass loss rate is 13.299wt%. EMR-doped specimens appeared three heat-absorption peaks, including the process of the loss of bound water by Mn(OH)₂ and Mn₃(PO₄)·6H₂O. NMR relaxation time T₂ spectra will be lagging behind, the pore size in the range of the transition pores and the distribution of the capillary pores. The total porosity decreases with the increase of doping, and the permeability decreases first and then increases. The pores of composites with 1wt% and 2wt% doping are mainly distributed by gel pores and transition pores, the distribution area of macropores is less, the internal structure is more dense, and the permeability is low, the saturation of bound fluid is high, and the saturation of free fluid is low.