The invention relates to a field in-situ monitoring device for the performance of a large scale farmland drip irrigation system and a diagnosis method. The monitoring device comprises: a monitoring device main body, a remote server and a user terminal; and the monitoring device main body is arranged in the drip irrigation system field for monitoring the inlet flow rate and pressure of the community, and the monitoring device main body adopts a sensing-transmitting-application object-connection architecture, which can remotely transmit the monitoring data to the remote server, and displays andanalyzes the monitoring results through a program run in the remote server. The management level of the system is greatly improved, and the problem of the lack of drip irrigation performance in-situ monitoring devices for large-scale farmland drip irrigation systems is solved. The system has the function of monitoring the water temperature, and can calibrate the flow rate obtained by the test to astandard flow rate of 20 DEG C through a temperature correction algorithm to eliminate the influence of the water temperature on the system performance, thereby more accurately and scientifically conducting diagnosis and assessment of the performance of the large-scale farmland drip irrigation system.