Full-field transmission X-ray microscope (TXM) has been widely applied in many research fields owing to its various strengths, such as in-suit non-destructive three-dimensional (3D) imaging with a nanoscale spatial resolution. 3D nano image beamline, a part of the Shanghai Synchrotron Radiation Facility (SSRF) phase-II project, focuses on cutting-edge scientific problems and national strategic needs. The main experimental methods are TXM and nano-computed tomography (nano-CT), and the energy range is 5-14 keV. The design goal for spatial resolution is 20 nm. Based on a bending magnet source, the beamline is built with a cylindrical collimating mirror, a double crystal monochromator, and a toroidal condenser. The experimental endstation adopts a self-designed and integrated full-field TXM system, and the mono-capillary condenser, the TXM mechanical system, and the nano-CT control and data acquisition software of the experimental endstation are all developed independently. The commissioning and performance tests of the SSRF 3D nano image beamline (BL18B) have been completed in 2021, with a resolution of 20 nm achieved for TXM imaging. This beamline is the first TXM beamline based on a bending magnet source in the world with a 20 nm imaging resolution. All the test results have reached the design goals for this beamline, and it will open to users in 2022.