The invention relates to a high-concentration N-phenylmaleimide reaction liquid post-treatment process, which is uniquely characterized in that the reaction liquid is sequentially subjected to water washing and neutralization, and the obtained liquid is subjected to solvent removing in a rotary evaporator to obtain the product without re-crystallization. The process comprising: adding water and a solvent mixture to a N-phenylmaleimide reaction liquid according to a certain ratio in a mixing manner, stirring, cooling, separating to obtain a water layer and an oil layer, carrying out water washing on the water layer, cooling the washed water layer, and carrying out vacuum suction filtration to obtain a maleic anhydride (short for maleic anhydride) by-product solid and water, wherein the maleic anhydride is used as the reaction raw material so as to be continuously used for the N-phenylmaleimide reaction step, and the water is continuously used for the water washing step; carrying out water washing on the oil layer, separating the oil layer to obtain a water layer and an oil layer, and distilling the neutralization water layer to obtain water and trace residue, wherein the water is continuously used, and the residue is treated; and adding the neutralization oil layer to the rotary evaporator, and evaporating the solvent to obtain the N-phenylmaleimide finished product with the purity of more than or equal to 99.25%. According to the present invention, the yield of the whole post-treatment process can exceed 97.5%, no wastewater is generated, and the process is environmental protection.