Based on the ERA5 reanalysis data, Doppler radar data and Beijing VDRAS data, a tornado with a supercell that occurred in Zhangjiakou on July 1, 2021, is analyzed. The results show that: (1) The tornado occured in the southeast quadrant of the cold vortex, the warm region in front of the shear line and near the surface convergence line. (2) The analysis of radar data shows that the tornado was generated on the south side of the supercell. During the tornado process, the height of more than 50 dBz was less than 6 km, and the strong core centre was less than 3 km, which was a convective system with a low centroid centre. In the wind field retrieved by radar, there was a closed cyclonic circulation at the lower level of 1 km. (3) The analysis of VDRAS data in Beijing shows that the strong convergence in the lower layer, divergence in the upper layer and the strong positive storm-relative helicity in the middle and lower layer provided favourable environmental conditions for the tornado occurrence. The vertical velocity distribution shows that there was strong upward movement and sinking velocity zone on both sides of the tornado. The vertical distribution of the disturbance temperature indicates that there was a negative centre below 4 km and a positive centre above 4 km.