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 and impingement distance on the evolutions of the impinging spray and the adhered film at simplified geometries with an expanding corner by employing a 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 the Rayleigh-Taylor instability theory. The film separation criterion is expressed as the 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. By integrating the improved model into the KIVA-3V code, the model was validated against the measurements and compared with the other models, and the differences between the improved and the other models were explored.