Provided are a locking device, a photovoltaic module mounting structure and a photovoltaic system. The locking device includes a blocking portion, a plug portion, and a guide portion. The blocking portion protrudes from the guide rail and is provided with a clamping groove for clamping a frame of the solar module. The plug portion is connected to the blocking portion, and the plug portion is configured to be in plug-in fit with the guide rail. The guide portion is connected to the blocking portion and provided with a guide ramp, one end of the guide ramp is flush with a notch of the clamping groove, and the other end of the guide ramp extends obliquely downward along a length direction of the guide rail.