The embodiment of the invention provides a fault diagnosis method and device for a brake lamp switch. The fault diagnosis method comprises the steps that the duration time of a first signal aiming atthe brake lamp end and a second signal aiming at the cruise end in this time of fault diagnosis process is obtained; when the duration time is greater than or equal to the preset time threshold value,first state identification and second state identification are obtained, and whether the first signal and the second signal are synchronous signals or not is judged according to the first state identification and second state identification; and if the first signal and the second signal are not synchronized, the brake lamp switch is subjected to fault diagnosis according to a preset fault diagnosis rule, under the situation that it is judged that the brake lamp switch is subjected to a fault, the function of the brake lamp switch is limited, and the brake light switch signal fault is reported. When the fault diagnosis method is applied for fault diagnosis, synchronization of the first signal and the second signal is judged, and when the first signal and the second signal are not synchronized, comprehensive diagnosis is conducted by combining parameters of the current vehicle speed, the acceleration, the braking pressure, the vacuum degree, the throttle opening degree and the like. Theproblem that the brake lamp switch signal is inaccurate due to a non-brake switch fault is effectively solved.