The invention discloses a virtual diagnosis method and system for an automatic transmission and aims at solving the problem that existing fault detection and diagnosis method cannot meet the requirement of high-efficiency and intelligent automatic transmission fault diagnosis. In the invention, function realization and failure conditions of each gearshift element are analyzed through power flow of each gear of the automatic transmission, and then the specific position of the faulted element of the automatic transmission is determined through a testing method. The diagnosis system mainly consists of a diagnosis platform and a testing analysis system, wherein the diagnosis platform comprises a PX1 controller, a CAN (Controller Area Network) bus interface card, a vehicle TCU (Transmission Control Unit) to be tested, a vehicle running state sensor and a roller bench for providing road running simulation; the testing analysis system comprises a power-flow-based diagnosis system platform constructed by LabVIEW2011. The design relates to the virtual diagnosis platform of the automatic transmission based on the power flow theory, which has the advantages of reasonable structure, simplicity and convenience in operation, reliable performance, high generality, low manufacturing cost and high accuracy, and can satisfy production and maintenance testing requirements.