The objective of this study is to gain an enhanced understanding of the characteristics of free spray development, mixture formation, and combustion under high-pressure split injection. The fuel was injected into a constant volume vessel by a single-hole nozzle with a hole diameter of 0.111 mm. The injection process comprised a pre-injection followed by the main injection. The main injection was carried out either as (i) a single main injection, with injection pressure of either 100 MPa or 160 MPa, or (ii) a split main injection with injection pressure of 160 MPa defined by the mass fraction ratio of 75: 25. A high-speed camera was used to observe the spray development under nonevaporation conditions. The tracer LAS technique was used to observe the spray mixture formation process. The diesel spray combustion and soot formation processes were studied using a high-speed video camera and examined by two-color pyrometry. The experimental results revealed that split injection can make the vapor phase more homogeneous. Split injection could enhance the combustion and decrease the soot emissions. The soot evolution process was divided into three parts under the single main injection, but into six parts during the split main injection. The effect of the split injection on the soot evolution process ended at the same time when the injection duration is same.