Underwater supersonic gas jet flows are sophisticated physical phenomena that widely exist in underwater propulsion, such as underwater rocket, high-speed torpedo, and water-piercing missile launcher (WPML). The present study aims to explore the characteristics of underwater supersonic jets. This research carries out two-dimensional and three-dimensional underwater jet experiments and establishes an improved spherical bubble model to study the transient and steady-state characteristics of the underwater supersonic jet. The simulation results show that the entrainment strength and anti-interference ability are the main factors affecting the transient characteristics of underwater supersonic jets. The experimental results show that the jet angle increases with increasing pressure and the increased rate of the jet angle decreases with increasing pressure. The jet pulsation exists in the entire experimental pressure range from 0.4 MPa to 6.5 MPa. The frequency of back-attack increases first and then decreases with increasing pressure in the chamber. The frequency peak appears at 0.8 MPa, which is 5.96 Hz.