Abstract: Diesel engines are required for the combination of low or zero-carbon renewable alternative fuels and high-efficiency combustion strategies, particularly with the increasingly stringent emission regulations and carbon dioxide (CO₂) emission limits. Methanol-diesel dual-fuel reactivity controlled compression ignition (RCCI) combustion can be expected to realize higher thermal efficiency and ultra-low nitrogen oxides (NO_x) and soot emissions. However, it is still lacking on the performance of the methanol-diesel dual-duel RCCI at high altitudes. This study focused on the combined effects of methanol substitution rate (MSR), main injection timing (MIT) and altitude environment on the combustion process, performance, and emission in the methanol-diesel RCCI engines. The parametric experiments were performed to change the MSR, MIT and altitude at 1 800 and 3 200 r/min. Firstly, the impacts of MSR at different altitudes (2 000, 1 000, and 0 m) on combustion, engine fuel economy and emissions were investigated under various engine speed and load conditions. A comparison was made with the conventional diesel combustion (CDC) running. Secondly, the effects of MIT at different altitudes on the combustion process, emissions and performance characteristics were experimented with, while the MSR was maintained constant. The results showed that the maximum in-cylinder pressure and peak heat release rate gradually increased with the increase in MSR, while the start of combustion (SOC) and CA50 were advanced. The equivalent brake specific fuel consumption (ESFC), the NOx and soot emissions were reduced significantly, whereas, the brake thermal efficiency (BTE), the THC and CO emissions increased at different altitudes. With the MSR increased from 0 to 20% at 1 800 r/min engine speed at 100% load, the maximum in-cylinder pressure increased by 1.72 MPa on average, the PHRR increased by 25.08 J/(°) on average, the ESFC decreased by an average of 4.67%, the BTE increased by an average of 4.90%, the NOx and soot emissions decreased by an average of 16.63% and 50% respectively, and the THC and CO emissions increased 142.03 mg/m³ and 388.18 mg/m³ on average at 0, 1 000 and 2 000 m altitude. With the MSR increased from 0 to 7% at 3200 r/min engine speed, the ESFC decreased by 1.76% on average, the BTE increased by an average of 1.79%, the NOx and soot emissions decreased by an average of 8.17% and 20.70%, respectively, at different altitudes. As the altitude increased from 0 m to 2 000 m, the PHRR was reduced by 4.80 and 8.08 J/°, the CA50 retarded 1.44° and 1.43°, the BTE dropped by 0.82% and 0.68%, the ESFC increased by 2.10% and 1.99%, the NO_x emissions decreased by 10.61% and 7.35%, the opacity smoke increased by 26.54% and 32.12%, the THC emissions increased by 29.88% and 15.45%, and the CO emissions increased by 22.42% and 18.15%, respectively, at 1 800 r/min with 20% MSR and at 3 200 r/min with 7% MSR. As the MIT was advanced while the MSR was kept constant at different altitudes, the maximum cylinder pressure and PHRR gradually increased, the CA50 was close to the TDC position due to advanced combustion phasing, BTE gradually increased, ESFC and soot emissions were reduced, and the NOx, THC, and CO emissions increased. As the MIT was advanced from -1.5° to -7.5° at 1 800 r/min with 15% MSR, the ESFC was reduced by approximately 8.27% on average, the BTE increased by 9.08% on average, while the opacity smoke decreased by an average of 90.94% at 0, 1 000, 2 000 m altitudes. The main injection timing of diesel fuel can appropriately increase to improve the thermal efficiency and fuel economy of methanol-diesel RCCI engines at high altitudes. This finding can provide a basis to optimize the control parameters for the better combustion and emission performance of methanol-diesel RCCI engines under altitude environments.