The invention discloses an optical path reuse beam splitter used for a Sagnac fiber optic interferometer. The optical path reuse beam splitter is composed of a total reflection bean splitting region, a tunneling region and a photonic band gap circulation region. A Y-type branch runs through the whole total reflection beam splitting region. The tunneling region has a tapered transition structure, and is used for guiding a light beam in the total reflection beam splitting region to enter the photonic band gap circulation region and preventing light in the photonic band gap circulation region from returning to the total reflection beam splitting region at the same time. A dielectric cylinder array covers the whole beam splitter structure, is used for reducing the bending and the leakage loss of the light beam in the total reflection beam splitting region and the tunneling region, and utilizes the photonic band gap effect for performing splitting and circulation guiding of the light beam in the photonic band gap circulation region. Compared with the existing beam splitter used for the Sagnac fiber optic interferometer, the optical path reuse beam splitter used for Sagnac fiber optic interferometer provided by the invention is characterized by one-way transmission, low loss, and capability of optical path circulation transmission, increases the light path without increasing the actual length of the light path, and greatly improves the measurement precision of the Sagnac fiber optic interferometer.