Abstract: A novel angle measurement method was introduced for measuring the deformation angle of large buildings and a photoelectric planar obliquity instrument was made. The speckle of laser was tested with a position sensitive device (PSD). In order to avoid environmental impact, magnetic fluid was utilized in the instrument. The viscosity of the special fluid could be adjusted by applying different magnetic field. Being reflected many times, the final angle of the laser beam was enlarged, which improved measurement precision. The design is simple, low cost, precise and robust to environment. The experimental resolution reaches 0.1 arc second and the repetition accuracy is better than 1 arc-second.