Abstract：Cyclobutane-1,2,3,4-tetracarboxylic dianhydride（CBDA）was continuously and efficiently synthesized by photodimerization of maleic anhydride（MA）in microreactor under UV irradiation. The influences of residence time of solution in reactor tube, tube size, reaction temperature and reactant concentration on product yield were studied, which showed that longer residence time, smaller inner diameter of the tube and lower reactant concentration were found to result in higher product yield. The yield and the unit time output of product could reach as high as 51.75% and 4.64g/h under reactant concentration of 0.15g/mL, reaction temperature of 5℃ and residence time of 80 min in the reactor tube with 2.00mm inner diameter. The photoreactor has better photon utilization efficiency, uniformity of irradiation, mixability and heat transfer performance compared to traditional batch photoreactor.