Objective: Currently, multi-element diffusion flames find applications in flame synthesis, combustion mechanism studies, and aerospace engine design. Therefore, investigating the characteristics of multi-element diffusion flames under both terrestrial gravity and microgravity conditions is crucial. In this study, numerical simulation methods are used to investigate the structural characteristics of ethylene-oxygen multielement laminar diffusion flames and the effects of pressure and oxygen volume fraction on the flame structure under both terrestrial gravity and microgravity conditions. Methods: This study was conducted using ANSYS Fluent software. First, a geometric model of the multi-element combustion chamber was constructed. The selected computational model included solving flow using a laminar model, diffusion using Fick's law, chemical reactions using a finite rate model, and radiation using a discrete ordinates model. Grid independence verification was performed, with 600, 000 grids chosen for calculations eventually. After the computational model was established, three sets of operating conditions were designed to study variations in flame behavior under different gravitational accelerations (0-9.8 m/s²), pressures (50-500 kPa), and oxygen volume fractions (0.25-1.00). The flame height obtained from the numerical simulation differed by less than 10% from the experimental results; thus, our method was considered to provide reliable results. Results: The results indicated that the flame had a double-layer structure. With decreasing gravity, because of the inhibition of buoyancy, the flame height increased from 7.3 to 12.8 mm, whereas the flame temperature decreased by 300 K. With increasing pressure, both the outer flame height and width decreased. At normal gravity, the temperature increased by 590 K, whereas it increased by only 80 K at microgravity. At 500 kPa pressure, the normal gravity fire separated, changing from a closed-tip flame to an open-tip flame at microgravity. As the volume fraction of oxygen decreased from 1.00, the flame height gradually increased. When it reached 0.50, the flame changed from a single flame to a double-layer one. Under normal gravity conditions, the flame temperature decreased by about 250 K, whereas it decreased by 600 K under microgravity conditions. Conclusions: The multi-element diffusion flame exhibited a double-layer structure under atmospheric pressure and fuel-rich conditions, with the inner and outer flames generated by the combustion of ethylene and CO, respectively. Meanwhile, modifications in pressure or oxygen volume fractions could change the shape from double-layer fire to separate flames or open-tip flame. The microgravity conditions enhanced the role of radiative heat transfer, leading to a significant decrease in flame temperature and eliminating convective mass transfer caused by buoyancy, thus increasing the flame height and width. Increasing pressure accelerated the reaction rate, increased the flame temperature, and reduced the flame height and width. Under microgravity conditions, increasing pressure enhanced the radiative heat transfer and lowered the flame tip temperature. Reducing the oxygen volume fraction reduced the flame temperature, increased the flame height, and converted the flame from separate flames to a double-layer flame, which was more susceptible to radiative effects and had a particularly low flame temperature in microgravity.