Film formation in spray-wall impingement process plays a critical role in engine performance and emissions. In this paper, the fuel film formation and the relevant film characteristics resulting from the liquid spray impinging on a flat plate were investigated in a constant volume combustion vessel by Refractive Index Matching (RIM) technique. The liquid film thickness was firstly calibrated with two different proportional mixtures (5 % dodecane and 95 % n-heptane; 10 % dodecane and 90 % n-heptane) pumped out from a precise syringe to achieve an accurate calibration result. The n-heptane fuel from a side-mounted single-hole diesel injector was then injected on a rough glass with the same optical setup, the distance between the injector tip and impinging plate is set to 33.65 mm. The ambient temperature and the plate temperature are set to 423 K with the fuel temperature of 363 K. The effects of various injector pressure (120 and 150 MPa) and ambient density (14.8, 22.8, and 30.0 kg/m3) on the liquid film properties were studied. The film properties contain the spatial distribution and time-resolved evolution of fuel film thickness, the film wetted area, and the film mass at the different operating conditions. The experimental results indicate that the fuel film behaviors are significantly influenced by the injection pressure but is insignificantly affected by the ambient density. The experiment in the present study is also served as the database of spray-wall interaction model development.