<p>Compression ratio (CR) is one of the technical tools for combustion control of marine low-speed engines. Due to the very large differences in ignition points and combustion speeds of different fuels, the CR has a large impact on the combustion, economy, and emissions of marine ammonia low-speed engines. This paper adopts numerical simulation to establish the in-cylinder combustion numerical model of marine ammonia low-speed engines and comprehensively analyses the influence of CR on ammonia combustion under different loads based on the model. The results show that a moderate increase in the CR helps gas mixing in the ammonia engine cylinder, and too large a CR worsens its combustion process instead. Following the CR of the prototype, NOx emissions from the ammonia-fuelled engine increase with increasing CR, with NOX emissions below the IMO. Tier-III value at high loads and above it at low loads. In addition, the ammonia engine knocking at 100 % load showed a tendency to decrease and then increase. With the increase of CR, the knocking was the smallest at CR of 23.8. The CR of the prototype was used at 100 %, 75 %, and 25 % loads, which resulted in poor overall combustion performance, and the optimum values of ammonia engine economy, emission, and knocking were reached at CRs of 23.8, 25.8, and 24.8, respectively, as a compromise between each other.</p>