Abstract: The objectives of this study are to evaluate the effects of different dosages and proportions of Mg(OH)2 and H3PO4 amendments on nitrogen conservation during simulated composting in laboratory. Ammonia volatilization was greatly reduced, ranging from 73.1% to 97%, and the nitrogen fixation ratio was improved with the increase of dosages and the proportion of H3PO4 of amendments. At the same time, the MgNH4PO4 could be formed during the process. The nitrogen conservation effect of the amendment of Mg(OH)2 : H3PO4 : H2O = 2.6:17.7:79.7 was the best. According to the analysis of benefit produced by the amendments, appropriate dosages could produce good benefit in practical application. As a result, several kinds of treatments were proposed to be the suitable amendments for reducing NH3 loss and improving the value of compost product during composting.