The invention discloses a method for detecting the harmful interfacial charge of a photovoltaic device with a mercury cadmium telluride film, and the method uses an ultrafast pulse laser beam that is provided with the photon energy larger than a device to be detected and has gaps for back-irradiating an HgCdTe device, uses a digital oscillograph for measuring the evolution relationship of the transient photovoltaic signal and the time between two electrodes of the photovoltaic device, and according to the fact that the photoemf that is measured by an external circuit and is formed by an electric field induced by the interfacial charge and a pn junction electric field is separated in the time dimension, the method identifies the photoemf which is formed by the electric field induced by the interfacial charge and the photoemf which is formed by the built-in electric field with metallurgical pn junction, thus acquiring the information of the interfacial charge which has great influence on the device photoelectric response more conveniently. The method is a convenient, quick and harmless identification method which has great important meaning for improving device performance, increasing device stability and guiding people to discover novel devices.