To improve the combustion stability of an ethanol engine under lean-burn conditions, the experiment was carried out on a spark ignition engine with ethanol intake port injection and hydrogen direct injection. To study the effects of ignition timing, excess air ratio, and hydrogen addition ratio on cycle-by-cycle variations of an ethanol engine, five different ignition timings, five different excess air ratios, and five different hydrogen addition ratios were applied at a low speed of 1500 revolutions/min and small throttle opening of 9%. The experimental results showed that the cyclic variation of the peak in-cylinder pressure decreased with the advance of ignition timing and the cyclic variation of the indicated mean effective pressure decreased first and then increased. When the ignition timing is fixed at 10 degrees crank angle before top dead center, the cyclic variation of the peak in-cylinder pressure, the peak pressure change rate, and the indicated mean effective pressure decreased with the increase of the hydrogen addition ratio. A 15% hydrogen addition can reduce the coefficient of variation of peak in-cylinder pressure by 75%. When the hydrogen addition ratio is 10%, the ethanol engine can run steadily under lean-burn conditions with an excess air ratio of 1.5.