Based on NCEP reanalysis data， surface observation data, Doppler radar data, wind profiler radar data, FY-4A satellite data and VDRAS data, the EF3 tornado event that occurred in Jinghai Area of Tianjin at 17:30 BT 13 August 2018 was analyzed. The results show that this tornado process was a strong tornado occurring in linear convection, and the convective storm causing tornado did not have the typical characteristics of supercell storm. After the formation of the tornado, on the basis of the continuous enhancement of the vortices, the center of rotation was declining, and the extension thickness of TVS was increasing. The formation and development of new convections between two linear convections promoted the merger of linear convections.There was a good correspondence between the grounding time of strong echoes of new convection and the occurrence time of the tornado. Twenty minutes before the tornado, the northerly wind on the north and west sides of the two convergence lines in boundary layers and the easterly wind constituted the convergent rotating flow field of the mesoscale low pressure. When the thunderstorm outflow met the local vertical shear center (up to 90×10-3 s-1 at the height of 180 meters), the uneven upward motion along the convergence line forced the horizontal vortex tube to tilt, forming a vertical vortex tube and being stretched in the vertical direction. According to the law of conservation of angular momentum, the strong tornado parent vortex was caused by the intensification of rotation. Thus, although the convective mode of non-supercell tornado in linear convection is different from that of supercell tornado, the physical mechanism of tornado parent vortex formation is the same.