Under the "dual-carbon" strategic goal, ammonia, as a carbon-free fuel, is one of the most promising options for the industrial and transportation sectors. The use of ammonia fuel in automotive engines can effectively reduce carbon emissions in transportation. This study investigated the combustion and emission characteristics of a gasoline/ammonia dual-fuel engine under different intake valve opening (IVO) conditions using a modified engine test bench and three-dimensional combustion simulation software. The IVO was adjusted within the range from −30°CA to 10°CA, with IVO = −356 °CA ATDC as the reference point. The results show that as IVO advances, the peak in-cylinder pressure and heat release rate gradually decrease, and the corresponding combustion phases are progressively delayed. This is due to the increase in residual gas in the cylinder as the valve overlap angle increases, which suppresses in-cylinder combustion. As IVO advances, the combustion phases CA10, CA50, and CA90 are initially delayed and subsequently advanced, due to the combined effect of valve overlap angle and the intake-exhaust pressure difference. The advance of IVO affects the intake valve closing angle, resulting in a gradual decrease in pumping mean effective pressure, which reaches its minimum value at IVO=−30°CA. Notably, the effect of IVO on thermal efficiency is small. Meanwhile, the emissions of unburned hydrocarbons, carbon monoxide, and nitrogen oxides increase gradually with the advancement of IVO, while the change in IVO has little effect on the emission rate of unburned ammonia. This is because the lower cylinder temperature reduces the oxidation of hydrocarbons and carbon monoxide, while the increased valve overlap raises the concentration of nitrogen oxides in the residual gas. Meanwhile, the decrease in pumping mean effective pressure (PMEP), which indicates reduced pumping losses, leads to a slight increase in the overall combustion temperature, thereby promoting the generation of nitrogen oxides. IVO influences the performance and emissions of a gasoline/ammonia dual-fuel engine. To improve emission characteristics under fixed exhaust valve timing, the IVO should be set to 10°CA. At this point, total hydrocarbons, nitrogen oxides, and carbon monoxide all reach their minimum values, but the brake thermal efficiency (BTE) is relatively low, at only 32.1%. To improve combustion characteristics, IVO should be adjusted to −30°CA. At this point, the BTE reaches 32.7%, but total hydrocarbons, nitrogen oxides, and carbon monoxide all reach their maximum values, leading to worsened emission characteristics. By using multivariate optimization methods to find the optimal valve timing combination, overall optimization of engine performance and emissions can be achieved.