The effects of compression ratio on combustion and unregulated emissions were researched on a glow plug assisted directinjection methanol engine retrofitted from DH1115 singlecylinder diesel engine by using Converge module of CFD software and the methanol detailed chemical kinetic mechanism. The results show that compression ratio has a significant effect on the combustion and unregulated emissions. Increasing compression ratio can improve the homogeneity of methanolair mixture due to the induced evaporation of methanol. A high compression ratio can advance ignition timing, shorten the ignition delay period, improve the ignition stability and promote the complete combustion of methanol. Formaldehyde and unburned methanol emissions decrease with the increase of compression ratio, but the drop amplitude decreases with the increase of compression ratio. The formaldehyde and unburned methanol emissions are less than 0.4 mg/(kW·h) and 3.1 mg/(kW·h) respectively when the compression ratio increases to 19 or higher.