Through laboratory experiments, this study examines velocities distribution within submerged vegetation patches under varying patch height-to-width ratios、volume fractions and submergences. Scale equations were adopted to the prediction for flow velocity adjustment length. The results showed that the flow velocity adjustment length within submerged vegetation patches was negatively correlated with the vegetation volume fraction and submergence ratio. Under identical height conditions, the length exhibited a positive correlation with patch width. The three-dimensional flow resistance factor was proposed under the influence of submerged vegetation patch. For high flow-blockage, flow velocity adjustment length was determined by patch width and height. For low flow-blockage, flow velocity adjustment length was determined by the patch frontal area per unit volume. Prediction formulas for flow velocity adjustment length were developed forth. These formulas demonstrated a high accuracy and yielded an average relative error of 10.3% with an average measurement uncertainty of 5.5%.