Static light scattering contains characteristic information of particle size, so static light scattering method is an effective means to measure the particle size of suspended matter in water body rapidly. However, the measurement accuracy of the static light scattering method for small particle size measurement is insufficient because the lateral and backward scattered light of particles is weak and not easily detected, and the forward scattering is affected by the Airy spot and there is a blind area for measurement. A low-level anisotropic scanning light scattering measurement method for small particle size suspensions in water is proposed, using a photomultiplier tube as the detector and a multi-angle continuous scanning method to detect the light scattering information of particles: by shortening the distance from the detector to the sample cell, the intensity of scattered light at the same angular resolution is increased, and the sensitivity of lateral and backward scattered light detection is improved; by deviating the detector from the excitation optical axis By avoiding the blind area of Airy spot, the forward large-angle scattered light detection can be achieved without changing the precision of the small-angle measurement of the front angle. On this basis, the method is combined with the meter scattering model to achieve the particle size measurement of small particle size suspensions. Experiments on different particle size samples show that the method can accurately measure the particle size of particles in the range of 350 nm to 2μm, and the relative errors of measurement of D50 for 2μm, 1.5μm, 500nm and 350nm specimens do not exceed 5.61%, all of which are lower than the relative errors of specimen uncertainty, and are better than the measurement results of laser particle size meters in the laboratory.