The invention discloses a flight path planning method and device for plant protection unmanned aerial vehicle, and can improve the operation precision and work efficiency of the unmanned aerial vehicle, reduce duplication coverage, spray leakage rate and energy consumption. The method comprises the following steps: fitting an operation area into a convex polygon, establishing a coordinate system, dividing corresponding area of the convex polygon by using grid to obtain a rectangular area composed of a plurality of raster regions; according to the rectangular area, acquiring an operation path of the plant protection unmanned aerial vehicle, and calculating the operation path length G; calculating the maximum path length Gm of a single operation of one plant protection unmanned aerial vehicle, calculating the midway return number of the plant protection unmanned aerial vehicle according to the G and Gm, and calculating efficient solutions of midway return points of each plant protection unmanned aerial vehicle corresponding to the midway return number; and selecting an optimal solution from the efficient solutions of coordinate of midway return point of each plant protection unmanned aerial vehicle, using the origin of the coordinate system as a starting point, and acquiring the flight path of the plant protection unmanned aerial vehicle according to the optimal solution and the operation path.