The invention relates to a device and a method for measuring the position and dimension of Gaussian beam waist. The device consists of a light path device and a circuit device, which is specifically as follows: a convex lens assembly and multimode fiber, a beam splitting plate erected on an optical fiber coupling bracket are sequentially arranged along the direction of reference light output by areference light source; the reference light is divided into transmitted light R (t) and reflected light R (f) after passing through the beam splitting plate; a first reflecting mirror is arranged in the direction of the reflected light R (f); a first part group and a second part group are sequentially arranged in the direction of the transmitted light R (t); the beam splitting plate, a polaroid sheet, a small aperture and a diode photodetector are sequentially arranged in the reflected light direction of the first reflecting mirror; and the circuit device includes a position detector signal processing circuit, a high voltage amplifying circuit, a triangular wave generating circuit, a plus or minus 15V operational amplifier power supply, an input socket and a computer with a PCI data acquisition board. The device and the method can greatly reduce the measurement cost and measurement error.