In order to analyze the effect of the double-spark plug ignition (DSI) on the heat release rate, cyclic variation in combustion and engine knocking, four different single- and double-spark plug ignition strategies are used for testing. Combustion diagnosis results show that when the DSI strategy is used, it can effectively promote the combustion process, increase the maximum combustion pressure, shorten the combustion duration and reduce the coefficient of cyclic variation (COV). Under low load condition, the effects are more pronounced.Test data reveals the following phenomenon. With the DSI strategy, both the cyclic variation in the early period of rapid-burning and in the later period of rapid-burning are reduced, especially in the early period of rapid burning. Then, the cyclic variation rate of the combustion duration is significantly reduced. Since the burning rate in the early period of rapid-burning is greatly accelerated, knocking tendency of the DSI engine is more obvious than that of the single-spark SI engine. The potential to improve engine performance and fuel economyby the DSI strategy largely depends on the optimization of the spark advance. Based on the optimization of the ignition timing of the DSI strategy S28&28, the break mean effective pressure (BMEP) is improved by about 5%- 6% compared to the original 158FMI engine.