Lean direct injection (LDI) combustion is one of the promising low pollution combustion technologies. One challenge of implementing LDI combustion is to achieve quick fuel-atomizing and mixing with air. In this paper, the spray characteristics and the prompt atomization process of the tangentially injected prefilming (TIP) LDI injector are investigated by experiments; the energy transmission model suitable for the prompt atomization process and the semi-empirical estimation model of SMD for the conical liquid film are derived. All experiments were carried out under 20°C and atmospheric pressure by using kerosene as the fuel. Firstly, effects of We and ALR on the spray distribution and SMD were studied by Mie scattering and Malvern spraytec laser diffraction system respectively, which were carried out at the operating conditions of We varying from 664 to 2656 and ALR varying from 16.3 to 24.5. Results show that large We is beneficial to disperse the spray in primary zone downstream the swirler. Then, breakup regimes of liquid film and droplets evolution were characterized by a high-speed camera with a long-distance microscope (LDM). Breakup regimes show that the film is torn up by the fierce outer airstream immediately. The primary breakup process does not rely on the surface waves anymore, which conforms to the prompt atomization mechanism. Finally, the energy transmission of the conical liquid film during the whole atomization process was analyzed, from which the semi-empirical estimation model of SMD was derived. The calculated and the measured SMD have good consistency, which demonstrates the applicability of the prompt atomization energy transmission model on a conical liquid film.