The invention is a device and measuring method for using Brillouin 
optical fiber circle 
laser to measure linewidth of ultra-narrow 
laser, which relates to a measuring device and method for 
laser linewidth. The output of the 
laser source is connected to the input of the 
isolator, the output of the 
isolator is connected to the input of the 
fiber coupler, the output of the coupler is connected to the inlet of the 
fiber coupler, the output of the 
fiber coupler is connected to the input of the photoelectric 
diode, the output of the photoelectric 
diode is connected to the input of the 
frequency spectrum analyzer, the fiber 
circular cavity is formed by connection one end of the aperture circle made up of unimodular fiber and the port of the 
fiber coupler, another end of the aperture circle is connected to the port three of the coupler. The method is: the laser needed to be measured is sent into the fiber circle cavity, when the power of the laser needed to be measured reach the threshold of SBS, it generates a first order Stokes light in the coupler, when the power of the first order stokes light reach the SBS threshold, it generates the second order stokes light in the coupler, then the second order stokes light and the laser needed to be measured frequency are analyzed by the 
frequency spectrum analyzer, and the linewidth can be acquired.