the present invention relates to a semi - aromatic polyamide and a method for preparing it with low wastewater discharge . the semi - aromatic polyamide for the present invention is obtained by introducing aromatic dicarboxylic acid , aliphatic diamine containing 4 ˜ 14 carbon atoms and the wastewater generated during the previous prepolymerization into an autoclave for prepolymerization reaction and then further polymerizing the prepolymer . in this preparation method , the wastewater generated during polymerization is recycled , thus greatly reducing the wastewater discharge ; the raw materials in the wastewater are effectively recycled , thus improving the utilization rate of raw materials ; meanwhile , the diamine in the wastewater compensates that lost along with water discharge during prepolymerization , thus ensuring the mole ratio balance between dicarboxylic acid monomer and diamine monomer .