This study focused on the application of a microbial biosurfactant as a synergist of the foliar fertilizer. A biosurfactant‐producing Bacillus velezensis MMB‐51 was firstly isolated, which produced the peak yield of 2.287 g/L biosurfactant BS‐51 at the optimum fermentation condition. The chemical nature of BS‐51 was identified as lipopeptide consisting of surfactin and fengycin, which exhibited a low CMC of 11.0 mg/L in ddH2O and a good stability in wide ranges of temperature, pH and NaCl concentration. The BS‐51 was then applied as a synergist of three common sweet potato leaf fertilizers. The result showed that BS‐51 increased the contact area of fertilizer with leaf surface by 51%–93% and the moving speed of fertilizer on the inclined leaf surface by 272%–459%, indicating that BS‐51 decreased the surface tension of three‐leaf fertilizers. BS‐51 also increased the fertilizer residue after it slipped from the inclined leaf surface from 18.5%–21.5% to 29.4–31.2% depending on their types, thereby improving the utilization efficiency of foliar fertilizer. In addition, B. velezensis MMB‐51 secretion inhibited the growth of sweet potato pathogen Ceratocystis fimbriata. Therefore, B. velezensis MMB‐51 could act as an efficient synergist of crop foliar fertilizer with antifungal activity.