Abstract: The insulation reliability problem under an extremely strong electric field is a key factor limiting the performance improvement of micro and nano electronic devices, so it is very important to understand the breakdown characteristics and the insulation failure mechanisms of nanogaps. In this paper, we utilize high-resolution imaging technology and nanoscale precision displacement technology to establish a vacuum nanogap pulsed breakdown test system based on transmission electron microscope and electrical sample holder, which can achieve accurate gap regulation with a spatial resolution of 1 nm. The effects of nanogap distance (5~25 nm) and applied voltage pulse width (100~1 000 ns) on the pulsed breakdown threshold of sphere-plate vacuum gap are studied. The results show that the relationship between pulsed breakdown voltage and gap distance is linear, that is, U_b=E_cd, and does not change with pulsed width. The breakdown field strength threshold is basically unchanged, about 8.8 GV/m. A smaller nanogap distance leads to a more concentrated electric field distribution and a smaller effective field emission region. The pulsed breakdown process of sphere-plate electrode under a uniform field is that the temperature of anode gold plate rises to nearly melting point under the energy deposition of cathode tungsten field emission current, and then thermal runaway occurs, resulting in electrical breakdown. This process, lasting approximately 25 ns, is independent of both gap distance and pulse width, which reflects the time required for vaporized atoms on the anode surface to form a plasma channel within the gap and to facilitate the cathode evaporation.