With societal and economic development, ammonia has become an important product in the chemical industry, gaining increased importance among energy sources as a carrier for hydrogen storage and transportation. As liquid ammonia transportation through pipelines generally occurs at medium to low pressures, the leakage process involves complex jet evaporation. In this study, the temporal changes in a vertically leaking liquid ammonia solution were simulated and the variations in velocity, temperature, and concentration fields in the near-field region of the liquid ammonia leakage were analyzed. The results show that when liquid ammonia leaks under 2 MPa and 20 °C from a 20 mm leakage hole to a 25 °C atmosphere, an ammonia cloud core forms at 17.54 m from the leakage hole in 1 s. By analyzing the changes in ammonia velocity, concentration, and temperature during the process, the leakage process of liquid ammonia can be divided into four stages: the initial leakage, air entrainment, cloud formation, and cloud dispersion stages. The liquid ammonia leakage process is more sensitive to changes in pore size and has a greater impact on the shape of the ammonia cloud. Pressure has a weaker effect on the leakage process and mainly affects the liquid ammonia jet velocity during the initial leakage stage.