In the distributed optical fiber sensor system, due to the difference of the manufacturer, materials, doping and the production process etc., the basic parameters of optical fiber sensor such as the refractive index, strain and temperature coefficient are slightly different, and it must be calibrated for sensing fiber before application. A method for calibrating the strain coefficient of optical fiber based on Brillouin Optical Time Domain Reflectometer (BOTDR) and stretching device was proposed, then the error analysis was presented. Several calibration experiments were conducted on G.652 single mode fiber with different initial length, and the strain coefficients calibrated were 0.050 2 ~ 0.051 2 (MHz/με), which showed good repeatability. Verified by BOTDR, the measured strain values agreed well with the results of theoretical calculation. The experimental results show that the calibration method is practical, easy to be implemented. It can be used both for the calibration of optical fiber sensing system and for the strain calibration of BOTDR in the production process.