[Aim]  Thosea sinensis is an important pest of tea. A simple quick and accurate method was developed to sex pupae and adults of T. sinensis to provide more accurate data for field population monitoring and prediction of this insect. [Methods]  The morphological characteristics of pupae and adult of T. sinensis were observed and compared. Adult genital types were used to validate the results of sexing. [Results]  The reproductive pore in female pupae is located on the 8th ventral segment and the oviposition hole is on the 9th ventral segment. The surroundings of these two holes are flat with no protrusions. The reproductive pore is on the 9th ventral segment and is characterized by a longitudinal crack shape. In male pupae there are semi-circular tumor-like protrusions on both sides of the reproductive pore. In female adults, the antennae are filamentous, and the end of the abdomen is cylindrical. Pressing the abdomen gently reveals that the oviposition valve is flat and oval. The antennae of male adults are comb-toothed, and the length of antennae is significantly shorter than that of adult females. In adult males, the end of the abdomen is thinner, and their reproductive organs feature a pincer-shaped grasper. The method can quickly identify the sex of adult and pupae of T. sinensis with 100% accuracy. [Conclusion]  Male and female T. sinensis pupae can be accurately distinguished by determining the position and shape of the reproductive pore. The sex of the adults can be accurately distinguished by observing the antennae or the genital structure by lightly pressing the abdomen.