The present disclosure relates to a brightness determining method and device, and belongs to the technical field of electronics. The method includes: acquiring a first brightness detected by a brightness sensor arranged on a backlight side of a display screen; determining a brightness area in the display screen, an orthographic projection area of a light receiving surface of the brightness sensoron the display screen being located in the brightness area; determining a brightness error value corresponding to the brightness area, wherein the brightness error value is positively correlated withthe brightness of light emitted from the brightness area; and determining the difference between the first brightness and the brightness error value as ambient brightness of a display side of the display screen. The brightness determining method provided by the present invention solves the problems that the screen-to-body ratio of a display device is relatively small and a display effect of the display device is relatively poor, increases the screen-to-body ratio of the display device and improves the display effect of the display device. The brightness determining method provided by the present disclosure is used for determining ambient brightness.