To investigate the influence of the flow channel’s width-to-narrow ratio on the Tesla valve’s flow characteristics, this paper establishes five Tesla valve models with different width-to-narrow ratios. Employing the laminar flow model, CFD methods were used to numerically simulate both the forward and reverse flows through the Tesla valve across a spectrum of width-to-narrow ratios, ranging from 0.2 to 1. The results reveal the flow trends in the straight or arc channels show opposite variation patterns when flowing in forward and reverse directions. The flow rate passing through straight channel is more sensitive to the response of a small width-to-narrow ratio. Irrespective of the flow direction, as the width-to-narrow ratio increases, the fluid flow within the valve demonstrates increasingly pronounced stratification. The pressure curve of the diversion section assumes an “∞” shape in forward flow, whereas in reverse flow, it assumes a “flat plate” shape.