In order to study the effects of methanol substitution ratio and excess air coefficient on the lean combustion and emission characteristics of a combined injection engine, a 3D simulation model was built based on a self-modified engine with an optical combined injection system comprised of methanol inlet injection and gasoline in-cylinder injection (MPI+GDI). The cylinder pressure tests were conducted and the in-cylinder combustion and emission characteristics were studied based on different excess air coefficients and methanol substitution ratios under lean combustion conditions. The results show that the flame propagation becomes slower, the peak of heat release rate appears later, the afterburning phenomenon increases, the peak of in-cylinder pressure decreases and the phase delays, and the indicated thermal efficiency tends to increase with the increase of excess air coefficient. Besides, CO and NO_x emissions reduce and unburned hydrocarbon (THC) emissions first decrease and then increase and reach the lowest at the excess air coefficient of 1.4 because the appropriate increase of excess air coefficient is beneficial to complete combustion and the great increase will lead to unstable combustion. With the increase of methanol substitution ratio, the peak of in-cylinder pressure increases and the phase advances, the fuel with a high methanol ratio burns faster, the mass center of combustion shifts forward, the exhaust temperature decreases, NO_x emissions increase, THC emissions first reduce and then increase, and CO emissions reduce. The results of study provide a theoretical basis for the optimization and design of methanol-gasoline combined injection engine.