A method of measuring focal ratio degradation of fibers based on a four-quadrant detector was proposed to improve the measurement accuracy of focal ratio degradation in fibers. The output spot of fibers was re-imaged on to the four-quadrant detector, and the spot position sensitivity of detectors was got through scanning the surface of the four-quadrant detector with a steering mirror, the actual size of the output spot and focal ratio degradation were measured indirectly in turn in this method. The relationship between the spot position sensitivity and the spot radius was deduced in detail. Meanwhile, the influences of the channel width of four-quadrant detectors on the focal ratio degradation were seriously studied.