Abstract: Combined composting of Agrocybe aegerita residue and Chinese herbal medicine residue often encounters problems such as slow temperature rise, long maturation period, and severe nitrogen loss. The effects and mechanisms of different nitrogen retention agents on the humification and nitrogen loss during residue composting remain unclear. This study conducted co-composting of Agrocybe aegerita residue and Chinese herbal medicine residue and added three different nitrogen retention agents including biochar (BAC), superphosphate (SSP) and calcium magnesium phosphate fertilizer (CMP) during the composting process, with no nitrogen retention agent as CK, aiming to explore the effects of different types of nitrogen retention agents on the maturity, humification process, and nitrogen loss of multi-material co-composting. The results showed that both CMP and SSP inhibited the temperature rise. The GI of the compost products from these two treatments was 33.06 and 19.57 percentage points lower than the control, respectively, but both GIs were still above 90%. BAC had no negative impact on the maturity of composting. Both BAC and SSP could inhibit the humification of composting. The HI of the compost products from these two treatments was 2.02 and 3.15 percentage points lower than the control group, respectively. The HR was 1.98 and 1.99 percentage points lower than the control, respectively. HA/FA ratio was 7.13% and 17.51% lower than the control, respectively. The HI and HR of the CMP treatment had no significant difference from the control group, but the HA/FA ratio was 1.72% higher than the control. The nitrogen retention effect of CMP treatment was the best. The nitrogen loss rate of the three treatments was 7.60 percentage points lower than the control, and the nitrogen fixation rate was 83.34 percentage points higher than the control. The order of nitrogen retention effect was CMP＞ SSP＞ BAC. The correlation analysis indicated that CMP promotes the formation of humus in composting by driving the degradation of organic carbon and chemical fixation of nitrogen through pH value, and synergistically regulating carbon and nitrogen conversion. The addition of calcium magnesium phosphate to the co-composting of mushroom residue and Chinese herbal medicine residue can enhance the humification degree, significantly reduce nitrogen loss, and improve the nitrogen fixation rate.