In order to understand the distribution characteristics of the muzzle temperature field for gun launched in different environments, numerical simulation and contrast analysis for 12.7 mm gun launched in air and underwater were carried out. With the FLUENT software, the numerical simulation was performed using User Defined Function (UDF) and dynamic grid technology. The numerical results show that when the gun is sealed and launched underwater, the velocity of projectile slows down just when it contacts water. At this time, the gunpowder gas after the projectile is slowed up and its temperature is raised, so that the initial injection temperature of the gas reaches 2653 K, which is 417 K higher than that in air (2236 K). Influenced by both the projectile movement and the gas-liquid interface, the Mach disk was formed at 70 μs after the projectile shot out. However, the Mach disk was initially formed at 200 μs after the projectile shot out for gun launched in air. Compared with the situation of gun launched in air, the shock core area formed by the underwater launch is significantly smaller, while the temperature of the core region is higher. Especially, the maximum temperature in the compressed wave is up to 3200K, which is 1500K higher than that in air. By fitting the curve of the Mach disk position with the time for the underwater launch, it is found that the variation of the Mach disk position follows the law of exponential change within 200μs after the projectile shot out.