Gasoline partially premixed combustion (PPC) is a potential combustion concept to achieve high engine efficiency as well as low NOx and soot emissions. But the in-cylinder process of PPC is not well understood. In the present study, the double injection strategy of PPC was investigated on a light-duty optical engine. The fuel/air mixing and combustion process of PPC was evaluated by fuel-tracer planar laser-induced fluorescence (PLIF) and high-speed natural luminosity imaging technique, respectively. Combustion emission spectra of typical double injection case were analyzed. The primary reference fuel, PRF70 (70% iso-octane and 30% n-heptane by volume) was chosen as the lower reactivity fuel like gasoline. Double injection strategies of different first fuel injection timing and mass ratio of the two fuel injections were comparatively studied. The results indicated that the combustion process of PPC was initiated by multipoint auto-ignition and these ignition kernels lied in the position where local equivalence ratio was relatively high. Then the flame fronts of these ignition kernels expanded to the region with lower fuel concentration. Jet/wall interaction, which hindered the fuel/air mixing of the first early fuel injection, was proved to have significant impact on the double injection PPC under low engine load. With an earlier first fuel injection timing and suitable mass ratio of the two fuel injections, the adverse impact of jet/wall interaction on fuel/air mixing could be reduced.