A system and method for online detection of a droplet deposition amount of a spraying operation of a plant protection machine, wherein the system is composed of parts such as a data processing module, a data collector, a temperature sensor, a spatial interdigital capacitive sensor, a communication module, a positioning module, a power supply module, and a control terminal, and wherein the spatial interdigital capacitive sensor is capable of outputting voltage data directly proportional to the droplet deposition amount; and provides a specific method and steps for detecting parameters of the droplet deposition amount, comprising: by the data processing module of the system for the online detection of the droplet deposition amount, acquiring the voltage data output from the spatial interdigital capacitive sensor via the data collector and processing the voltage data, and transmitting the processed voltage data to the control terminal via the communication module.