Abstract: To investigate the mechanism of screech tone generated by supersonic underexpansion jet, a rectangular contraction nozzle jet simulation experimental device with an aspect ratio of 10∶1 was used. A combination of far-field noise testing and near-field flow schlieren measurement was used to study the unsteady motion of the supersonic jet shock cell structures and its interaction with large-scale vortices to generate screech tones. The experimental results show that there are different generation mechanisms for screech tones in the upstream direction and downstream direction. The screech tones in the upstream and side directions of the jet are mainly the arc-shaped pulse wave formed by the interaction between the large-scale vortices and the shock cell tips, while the screech tones in the downstream direction are mainly caused by Mach wave radiation. The experiment revealed the unsteady motion law of the shock cell structures in supersonic jet flow, and discovered the arc-shaped pulse wave property of screech tones. In addition, the Gaussian-modulated sinusoidal pulse train has been used to simulate the arc shaped pulse waves and the results showed that the pulse nature of screech tones and the multi-source mechanism can lead to the generation of higher-order screech tones.