The invention discloses a device for measuring system delay time in the variable spraying process based on a real-time sensor. The device is mainly composed of a photoelectric sensor, liquid level photoelectric sensors, a signal capture timer, a sliding block, a servo motor with the adjustable speed, a servo motor controller, a sliding rail platform and the like, wherein the photoelectric sensor is installed on a spraying machine, the photoelectric sensor and the original real-time sensor are in the same perpendicular position, the liquid level photoelectric sensors are fixed to nozzles and used for sensing water, and the sliding block is used for stimulating moving plant targets. The interval between time when the plant targets are detected by the real-time sensor and time when spraying operation is started is the system delay time in the spraying process, the delay time is inherent in a system, and it is important to accurately measure the delay time. In the spraying process, the real-time sensor is installed in front of the nozzles by a certain distance, the plant targets are sensed in advance, time needed for the plant targets to reach the nozzles is calculated through the real-time speed, and in combination with the measured system delay time, accurate targeting spraying can be achieved.