Small angle neutron scattering (SANS) is a coherent elastic scattering technique. It uses neutron as the probe to characterize the microscopic and mesoscopic structures of materials. SANS has been widely used in the fields of physics, chemistry, material science, biology and so on. At the early stage, SANS used pinhole geometry to collimate the neutron beam, and measured the length scales from 1 nm to 100 nm. With the increase in the neutron flux and the development of the neutron optics and detection technologies, SANS gradually develops into two directions, i.e., larger length scale, and smaller length scale, respectively. To detect larger length scale, Very Small Angle Neutron Scattering (VSANS), Ultra Small Angle Neutron Scattering (USANS) and Spin Echo Small Angle Neutron Scattering (SESANS) are developed; to detect smaller length scale, Neutron Total Scattering instrument for Disordered Macromolecules is developed. Nowadays, SANS is aiming at full-scale coverage and intelligent data analysis, hoping to measure the multi-scale structures of complex systems at one time. This article follows the development history of SANS technologies, explores its development direction and intersperses its role in the field of basic and applied scientific research.