The invention provides a magnetic field sensor based on a photonic crystal plate microcavity. The magnetic field sensor comprises a photonic crystal plate and a magnetic sensitive film; the photonic crystal plate is provided with a plurality of air holes formed in an array mode; the photonic crystal plate is provided with a point defect microcavity, and the surface of the microcavity is covered with the magnetic sensitive film; and photonic crystal waveguide is arranged on the photonic crystal plate. According to the magnetic field sensor, the surface of the photonic crystal plate microcavityis covered with the magnetic sensitive film, and the refractive index of the magnetic sensitive film changes to different degrees with the change of an external magnetic field; due to the coupling effect of evanescent waves, the energy loss in output spectrum of the photonic crystal plate waveguide changes; the change amount of the external magnetic field is reversely deduced according to the sizeof the energy loss; and the magnetic field sensor has the excellent characteristics of safety, explosion prevention, electromagnetic interference resistance, light weight, fast response speed, largemeasurement range, real-time remote detection and the like, the problem that a traditional optical magnetic field sensor has low sensitivity in measuring weak magnetic fields is solved, and the measurement precision of the weak magnetic fields is improved.