Provided are a device and method for automatic diopter adjusting. The device and method are applied to VR glasses and achieve a desirable automatic VR diopter adjustment effect. The device includes: a barrel holder, a display module, a diopter adjusting lens assembly, at least two guide slots, an adjusting ring, a processor, a pupil image acquisition unit, and a transmission assembly. The pupil image acquisition unit acquires a pupil image displayed by the display module and transmits the pupil image to the processor. The processor determines whether diopter adjustment is needed, generates adjustment driving information, and drives the transmission assembly according to the adjustment driving information to work. The transmission assembly drives the adjusting ring to rotate, thereby driving the diopter adjusting lens assembly to rotate to adjust a distance between the diopter adjusting lens assembly and the display module, and realize adjustment on the diopter.