In the field of shooting range testing, the trigger is mainly used to detect the physical phenomena generated when the projectile is fired, and output the trigger signal at the zero time of the projectile firing to start different subsequent ballistic testing equipment to achieve synchronization of multiple parameters. Existing triggers are easily affected by the environment, and are difficult to handle due to their large size, and have relatively large usage limitations. In order to solve the above limitations, the infrared radiation of the flying projectile in the terminal ballistic section is studied in this paper. A suitable mid-wave infrared detector is selected. A mid-wave infrared trigger is designed. The mid-wave infrared signal is gathered through an optical structure, and is focused on the PbSe detector. Corresponding signal processing circuit is designed on the photosensitive surface to realize accurate detection of projectiles. Tests have verified that the trigger can effectively detect projectiles. It has high detection sensitivity, and has good environmental adaptability and reliability.