The invention discloses a device for detecting the geometric outline of a cavity through borehole camera shooting. The device comprises a digital camera, a conical lens, a photosensitive element, a data line, a shell, a computer and an orientation detecting instrument. The invention further relates to a method for detecting the geometric outline of the cavity through borehole camera shooting. The method is suitable for testing the geometric outline of the drilling cavity of underground construction rock. According to the method, the digital camera in the device is used for carrying out focusing operation on any selected point, the distance of detection points of the cavity wall is calculated through a lens imaging formula, and the space geometric form of the cavity is measured. Through the device and method, the size of the form of the cavity can be detected in the borehole omnidirectional imaging process.