In order to solve the problems of uneven grain size and the pipe blockage during the crystallization process of oxalic acid, the vacuum flashing crystallization technology was used to the crystallization process of oxalic acid. The vacuum flashing crystallization system is more stable and the oxalic acid crystal particles is larger and  uniform through the reasonable control of the process parameters, the vacuum degree is 90.4 kPa, the crystallization temperature is 50 ℃, and the average residence time is more than 3 hours. The results show that the vacuum flashing crystallization technology can be used successfully in the crystallization of oxalic acid, and has the advantages of simple process, low equipment investment, no cooling water, low operating costs, and  no easy  scaling compared with traditional cooling crystallization technology.