The invention discloses a method and a system for detecting a device fault. The method includes: 1, acquiring model parameters and a rule base, wherein the model parameters include a device type, a device structure, the number of measuring points, the distribution of the measuring points and the attributes of the measuring points, and the rule base includes fault types, corresponding fault characteristic parameters and set thresholds; 2, constructing the fault models of a device according to the model parameters and the rule base, wherein every device type corresponds to multiple fault types, and every fault model comprises the fault characteristic parameters, the set thresholds of the fault characteristic parameters, and the corresponding fault types; 3, acquiring the infrared thermogram of every measuring point of the device; 4, extracting the device characteristic parameters from the infrared thermogram according to the model parameters; and 5, acquiring the fault detection result of the device according to the fault models and the device characteristic parameters, wherein the fault detection result comprises the fault location and the fault severity level. The method and the system have the advantages of realization of the analysis of the running state of the device, realization of online detection, and improvement of the detection accuracy of the device faults.