Gravity currents are important in many fields, including the estuarine sciences, meteorology and hydraulic engineering. The NHWAVE (non-hydrostatic wave) model was applied to simulate the detailed interface structure between a lock-release gravity current and the ambient fluid. The simulated structures, including the front height, front position and velocity of the current, are consistent with the results of laboratory experiments. However, the internal structure of the current is different from that revealed by previous research. The Kelvin–Helmholtz phenomenon in the interface and the interface vortices were successfully captured by the NHWAVE model. The difference in velocity between the front and rear vortices leads to entrainment, further causing changes in the shapes and amount of vortices. Flow field results obtained by the NHWAVE model reveal the existence of a significant circular flow, as well as some small eddies within it. The significant circular flow supports the forward movement of the current, whereas the small eddies reflect interface vortices. In contrast, hydrostatic simulation with the same model settings fails to capture the vortices. This research shows that the NHWAVE model performs better than a hydrostatic model when simulating the Kelvin–Helmholtz instability phenomenon and vortex entrainment in a lock-release gravity current.