Studying the combustion stability of engines equipped with a direct water injection system, especially under high compression ratios and load conditions, is important for realizing the stable operation of downsized boost engines with high thermal efficiency. This study implements a downsized single-cylinder engine with a direct water injection system to systematically investigate the combustion stability under a high compression ratio (CR = 17) and load (IMEP = 9 bar). According to the study results, when the water mass increases, the increase in combustion duration and retardation of the combustion phase at a fixed spark timing lead to more severe fluctuations of cycle-to-cycle variations in the indicated mean effective pressure. This research also indicates that water direct injection advances spark timing to increase power capability while maintaining better combustion stability. It can extend the knock limit spark advance to varying degrees when the water/fuel ratio increases from 0.1 to 0.5. In addition, the coefficient of variation in the indicated mean effective pressure drops to 1.15% with knock limit spark advance when the water/fuel mass ratio is up to 0.5.