An engine experimental platform with gasoline port injection and hydrogen/gasoline direct injection was built to study the effects of combined injection mode and ignition timing on combustion and emission characteristics of a spark-ignition engine. The study was performed in three modes, including gasoline port injection (GPI) mode, gasoline port injection plus gasoline direct injection (GPI + GDI) mode, and gasoline port injection plus hydrogen direct injection (GPI + HDI) mode, under the conditions of a constant excess air ratio and speed. Through the comparison and analysis of the three modes, the results reveal that combined injection is beneficial to increasing brake power and decreasing brake-specific fuel consumption and hydrocarbon (HC) and carbon monoxide (CO) emissions. The combustion process will be accelerated and concentrated with combined injection as well as hydrogen addition, and the effect of adding hydrogen is stronger than that of gasoline direct injection. Brake thermal efficiency and brake power increase, HC and CO emissions decrease, and nitrogen oxides (NOx) emission increases by adding hydrogen. In any mode, brake power and brake thermal efficiency first rise then fall as the ignition advance angle increases. HC and NOx emissions decrease by retarding ignition.