Flow distortion generated by L-shaped elbows in plant-protection spraying equipment disturbs the inlet flow of flat-fan nozzles, causing non-uniform outlet velocity distributions and reduced foliar deposition uniformity. This study developed a flat-fan nozzle with built-in flow-straightening vanes to improve spray stability under complex pipeline conditions. A three-dimensional CFD model integrating an L-shaped elbow, nozzle, and external spray region was established, and the Volume of Fluid (VOF) model was used to examine the effects of flow-dividing channel number, vane geometry, and vane insertion depth on velocity distribution on the spray fan plane. Structural parameters were optimized using a Box–Behnken response surface design. Field experiments on soybean seedlings evaluated droplet deposition using a carmine tracer assay with microplate reader measurement. The built-in vanes reduced elbow-induced flow deflection, swirl, and localized high-velocity zones, thereby improving spray fan velocity uniformity. The influence on the coefficient of variation of normal velocity followed the order: vane geometry > insertion depth > channel number. The optimal configuration comprised four channels, a star-shaped vane, and a 10 mm insertion depth, yielding a normal-velocity coefficient of variation of 16.12% at 400 mm downstream of the nozzle. Field trials showed that the developed nozzle reduced droplet deposition CV from 4.46% to 2.03% compared with the conventional flat-fan nozzle, a 54.5% decrease, with a significant difference between nozzles (p = 0.006). These results indicate that built-in flow-straightening vanes can improve spray stability and foliar deposition uniformity under elbow-induced flow distortion.