The phenomenon of a thin liquid film separation and atomization at expanding corners during the spray/wall interaction is usually encountered in premixed charge compression ignition (PCCI) engines. However, detailed information about the film separation is very limited, especially under high injection pressure conditions. In this study, experimental study was conducted to investigate the effects of injection pressure, corner angle, injection angle, and impingement distance on the evolution of the adhered film at simplified geometries with an expanding corner by employing the high speed camera. In addition, an improved hybrid film separation and atomization model was developed, which includes the sub-models of film separation criterion, film separation mass ratio, and the film atomization model based on Rayleigh-Taylor instabilities. The film separation criterion is expressed as the force ratio of the inertial, surface tension, and gravitational forces around the corner, and the film separation mass ratio is a function of the force ratio by curve-fitting the corresponding experimental data to well reproduce the partial separation phenomenon. The improved model was integrated into the KIVA-3V code, and the differences between the improved and the other models were explored in detail.