The effect of UV-B radiation on plants is reflected in multiple levels. DNA damage is caused by UV-B radiation, which leads to abnormal mitosis and ultimately affects plant growth and physiological and biochemical processes. RAD21.3 is a subunit of cohesin complex, which is involved in chromosome separation during mitosis. In this paper, Columbia-0 Arabidopsis thaliana and atrad21.3 were used as materials, and the control and UV-B treatment group were set up to analyze the root length, plant height, bolting time, physiological and biochemical parameters of WT, atrad21.3 and overexpressed transgenic plants. The mitosis of Arabidopsis root tip cells was observed by basic fuchsin staining, and the aberration rate was counted. After UV-B treatment of the WT and atrad21.3 mutants, it was found that the UV-B treated-WT and atrad21.3 had similar bolting time, plant height and various physiological and biochemical indexes. Through the construction of expression vector, the results showed that RAD21.3 was located in the nucleus. Further observation of mitosis revealed abnormal phenomena such as lagging chromosomes, chromosomes bridge, fragments chromosomes, etc. Statistics show that the aberration rate of the UV-B treated-WT is similar to atrad21.3, and the aberration rate of the UV-B treated-atrad21.3 increases. The above results indicate that RAD21.3 may respond to abnormal mitosis induced by UV-B radiation.