A flashlight with lateral locking function for firearms, comprises: a flashlight body and a lateral locking mechanism, the lateral locking mechanism is used to fix the flashlight body to a firearm; the lateral locking mechanism comprises a bottom box and a connecting component, an adjustment component is arranged on the bottom box, the adjusting component is connected with the connecting component, and comprises an adjusting handle, the adjusting handle is connected with a side wall of the bottom box, using to drive the movement of the connecting component, so that the connecting component is connected or disconnected from the firearm. The disclosure realizes the quick installation and disassembly. The whole installation process does not need to use other tools such as wrenches, and the installation is firm; the adjusting handle is arranged on the side wall of the bottom box, which is convenient for users to operate.