The stepwise principle of the step projection lithography machine is used to control the exposure dose to prepare a wedge-shaped mode size converter.The effects of different exposure doses on the sidewall morphology and the best etching parameters were analyzed. The experimental results show that the optimum exposure time is 20 ms each time, the reflow temperature is 160 ℃ for 1 min. When the etching gas and the ratio are SF6∶He=8∶80, the sample obtained after etching has good morphology. The preparation method has the advantages of short manufacturing cycle and high precision, and has the advantages of high efficiency and low cost compared with the electron beam gray exposure method.