The invention provides a device and method for measuring the line width of an optical cavity. The device comprises the optical cavity, a single-frequency laser device, a single-frequency tunable laser device, a photoelectric detector and a spectrum analyzer. By the adoption of the device, the method for measuring the line width of the optical cavity includes the steps that output lasers of the single-frequency tunable laser device and output lasers of the single-frequency laser device are coupled on a first optical beam splitter; combined beam lasers are divided into two beams through a second optical beam splitter, one beam of lasers are guided into the optical cavity, and the other beam of lasers are guided into the photoelectric detector; the combined beam lasers are adjusted to be matched with the optical cavity in mode; output lasers of the optical cavity are input into the photoelectric detector; the wavelength of the single-frequency tunable laser device is changed, a set of frequency and noise power of corresponding beat frequency signals are recorded through the spectrum analyzer; the line width of the optical cavity is obtained according to formula fitting. The device is simple, convenient to operate and accurate in result and has excellent practical value.