The invention relates to a field fault diagnosis system. An acquisition operation and maintenance management platform issues a task to a mobile operating terminal to perform association of the task and the fault type; and fault detection is performed by connecting a field detection device through the task fault type so that the requirement for operation and maintenance personnel can be reduced. Communication of data interaction of the whole system can be performed in a wireless manner so that the constraint of cables can be eliminated, operation is simpler and more convenient, the field is cleaner and the influence on the operating efficiency caused by various problems due to the cables in the operation process can be avoided. The field detection device is internally provided with intelligent and automatic detection for multiple fault types; and the detection result is associated with the task to be automatically transmitted back to the acquisition operation and maintenance management platform so that task fault datamation and reasonable statistics and classification can be performed. The field detection device can support hardware detection of multiple acquisition modules and parameter detection and setting of acquisition field equipment. The separate design is adopted for the field detection device, and the field detection device can be separated from a conversion plate so that the expansibility can be enhanced without carrying multiple sets of equipment in field testing.