The invention discloses an unmanned aerial vehicle (UAV) based method for measuring particle size and distribution of river bed surface boulders. The main technical scheme is characterized in that thecenter axis particle size Z values (3) of the boulders are measured on site; the flight height of the UAV and the overlap degree are determined through requirement on the space resolution of the Z values; and finally, orthographic projection of the UAV is corrected and calculated by using relevant software so as to obtain the three-dimensional digital elevation data of a river bed surface, and plane particle size distribution of the boulders, the particle size grading curve and the surface roughness height are obtained in the end. According to the Z values of the boulders, an RTK (Real-time kinematic, real-time dynamic) carrier phase difference technology is needed to be adopted and in-situ accurate positioning is carried out to obtain certain characteristic elevation points, and the characteristic elevation points are used for correcting particle size distribution of the boulders. RTK positioning points are marked through drawing circles on a river bed by lime powder to ensure that images provided by the UAV are easy to identify. The particle size of the boulders aerially measured by the UAV is verified by selecting samples of the center axis particle sizes of the boulders in a small-range field, so that the particle size and distribution of the boulders in a research area can be measured.