The invention relates to a four-quadrant-detector-based linear optical current sensor and a detection method thereof. The linear optical current sensor comprises a laser source, a polarizer, a magneto-optical film, a beam expander, a radial polarization detection grating, a four-quadrant detector and a signal processing system that are arranged successively at one same optical path. In addition, the linear optical current sensor also includes a magnetic collection ring with air gaps; a to-be-detected current-carrying lead passes through the magnetic collection ring; and a magneto-optic thin film is arranged between the air gaps. In addition, the detection method includes: an optical signal passes through the polarizer, the magneto-optical film, the beam expander, and the radial polarization detection grating successively from the laser source and then a light spot is obtained; after the light spot passes through the four-quadrant detector, the optical signal is transformed into an electrical signal; the signal processing system collects the electrical signal and operation is carried out to obtain the size of a rotation angle, so that a to-be-detected current value in the to-be-detected current-carrying lead is obtained. Compared with the prior using the CCD camera, the sensor and the detection method enable the resolution ratio to be improved substantially, the accuracy to be enhanced, and the cost to be reduced.