The combustion characteristics of gasoline-diesel dual-fuel in an electronic-controlled high pressure common rail optical engine were investigated under different diesel injection timings and gasoline/diesel ratios by a high-speed photography method. The experimental results show that the dual-fuel combustion process is influenced by diesel combustion and gasoline homogenous combustion, respectively, with bright yellow flames and blue flames observed in the combustion chamber. At a gasoline/diesel ratio of 0.91, the injection timing affects the ignition timing and combustion modes significantly. When the diesel injection timing is before −25° after top dead center (ATDC), advancing the injection timing tends to prolong the ignition delay and the gasoline-diesel dual-fuel combustion is similar to the pre-mixed charge compression ignition (PCCI) combustion with a rapid single-stage heat release. When the diesel injection timing is after −25° ATDC, as the injection timing is advanced, the ignition timing is also advanced and the dual-fuel combustion takes place in two stages.When the injection timing is at −6° ATDC, the gasoline/diesel ratio has a strong influence on the ignition location and the proportion of premixed combustion. With an increase in the gasoline/diesel ratio, the ignition spots mainly appear around the diesel plumes and concentrate close to the center of the combustion chamber. The proportion of premixed combustion is enhanced during the entire combustion process. The injection timing and the gasoline/diesel ratios both control the combustion phasing and pressure rise rate effectively.