In view of the problems that optical fiber MHD magnetic field sensor is susceptible to environmental in- terference and the MHD film production process is complex , laser self-mixing interference has the characteristics of strong anti-interference ability and good stability , a magnetic field measurement method based on spatial light horizon- tal polarization laser self-mixing interference is proposed. In a certain range , the magnetic field is proportional to the change of the signal amplitude , and the magnetic field is detected by using the mean ratio of the signal amplitude be- fore and after the magnetic field is applied. The experimental results show that the amplitude of signal attenuates obvi- ously in MHD with 1% concentration , and the film thickness is 0. 5 mm , which reduces the difficulty of making the film. In the experimental results , the mean ratio of 5. 02 mt to 19. 112 mt magnetic field intensity was linearly fitted , and the relative error was at least 0. 17% .