The invention provides a method and a device for detecting the operating performance of an air preheater. The method comprises the following steps of: simplifying the air preheater into a first heat exchanger and a second heat exchanger according to a theoretical simplification model of the air preheater; acquiring a first operating parameter of the first heat exchanger and a second operating parameter of the second heat exchanger; obtaining a first characteristic parameter of the first heat exchanger in a test state according to the first operating parameter, and obtaining a second characteristic parameter of the second heat exchanger in the test state according to the second operating parameter; obtaining a first performance parameter of the first heat exchanger in a boundary state according to the first characteristic parameter, and obtaining a second performance parameter of the second heat exchanger in the boundary state according to the second characteristic parameter; obtaininga performance parameter of the air preheater according to the first performance parameter and the second performance parameter; and determining the performance state of the air preheater according tothe performance parameter of the air preheater and a preset parameter threshold value. The method and the device for detecting the operating performance of the air preheater provided by the inventionrealize the quantitative detection of the performance of the air preheater.