A venturi mixer prototype is developed for mixing air and synthesis gas or "syngas" as a fuel. Syngas is recognized as a viable energy source worldwide, particularly for stationary power generation. It has a very low energy density, so a mixer with ${lambda}$ (ratio of actual to stoichiometric air-fuel ratio) in the range of 1.1 to 1.7 is expected. In this study, three-dimensional computational fluid dynamics (CFD) modeling is used to investigate and analyze the influence of the throat diameter, gas chamber thickness and gas exits diameter on mixer characteristics and performance. Attention is focused on the effect of venturi mixer geometry on the air-fuel ratio, pressure loss and mixing quality. Based on the numerical results, an optimized design of venturi gas mixer is made. The optimized design has ${lambda}$ in the range of 1.2 to 1.3, and gives very good mixing quality and acceptable pressure loss. The CFD results are validated with experimental data. The CFD results show good agreement with experimental data.