The invention discloses a method for limiting the speed of vehicles and a vehicle-mounted device. The method comprises the following steps: when a preset first speed limit time point is reached, a main control unit sends a semi-speed limit command, and an execution unit receives the semi-speed limit command, rewrites normal engine parameters during the last starting of an engine into abnormal engine parameters according to the semi-speed limit command when the engine is started or rewrites the abnormal engine parameters into the normal engine parameters and provides the rewrote engine parameters for an engine control unit; when a preset second speed limit time point is reached, the main control unit sends a full-speed limit command, and the execution unit receives the full-speed limit command and provides the abnormal engine parameters for the engine control unit according to the full-speed limit command when the engine is started. Due to the adoption of the method for limiting speed of vehicles and the vehicle-mounted device, a semi-speed limit cushioning period can be set, the purpose of reminding owners for repayment is achieved, and the vehicles can complete current transport tasks.