The invention discloses a transformer oil state monitoring expert system which comprises a state monitoring conditioning circuit board, a reasoning diagnosis terminal module based on an ARM structure and a chief control module based on a server framework. The state monitoring conditioning circuit board is electrically connected with the reasoning diagnosis terminal module and transmits collected transformer oil state information to the reasoning diagnosis terminal module; the reasoning diagnosis terminal module is electrically connected with the chief control module; in the actual operation, the reasoning diagnosis terminal module utilizes rules in a knowledge base to carry out reasoning analysis calculation in a terminal and sends the calculation results to the chief control module; and the chief control module retrieves the calculation results from the reasoning diagnosis terminal module automatically and stores the calculation results to a database. The expert system not only can greatly reduce equipment failure rate in a maintenance period and provide technical basis for equipment state maintenance, but also can find equipment defects and abnormal symptoms in time, thereby ensuring safe operation of the equipment, and improving power supply reliability.