A syngas/diesel dual-fuel reactivity-controlled compression ignition (RCCI) engine was numerically investigated by an improved multidimensional model coupled with a reduced chemical mechanism. In the test RCCI engine, the syngas was premixed with air in the intake manifold, while the diesel was directly injected into the cylinder well before top dead center (TDC). The effect of the syngas composition, the premixed ratio of the syngas, the initial in-cylinder temperature at intake valve closing (IVC), and the hydrogen (H-2) proportion in the syngas on the RCCI combustion and emission characteristics were investigated. The results indicate that the utilization of the syngas/diesel dual-fuel strategy in the RCCI engine with lean and premixed combustion is capable of simultaneously reducing the emissions of nitrogen oxides (NON) and soot. Compared with the gasoline/diesel RCCI combustion, the combustion characteristics of the syngas/diesel RCCI is much more complicated due to the complex composition of syngas, which plays an important role in the ignition and combustion processes. The H-2 in the syngas inhibits the autoignition of the RCCI combustion and significantly affects the heat release process, while the inclusion of carbon monoxide (CO) in the syngas is beneficial to mitigate the rapid combustion rate of H-2. Consistently, the addition of the inert gases (e.g., N-2 and CO2) decreases the global heat release rate and ringing intensity, whereas excessive inert gases in the syngas lead to incomplete combustion and low fuel efficiency. In this study, the optimal solution with the syngas premixed ratio of 60% and the H-2 volume fraction of 75% in the syngas can achieve RCCI combustion with both high fuel efficiency and low emissions.