The invention designs a particle size measurer based on a single photoelectric detector; the particle size measurer comprises a DMD (digital micromirror device) and control circuit, a Fourier transform lens, a converging lens, and a single photoelectric detector; a method comprises the steps of expanding a laser into parallel beams to irradiate particles under detection so that diffraction occurs, converging by the Fourier transform lens, and allowing diffracted light to irradiate a DMD on a focal plane; deflecting micromirrors of the DMD to reflection state, collecting reflected light by the single photoelectric detector, and determining a position with greatest intensity of reflected light as a diffractive spot center; from the diffractive spot center, arranging the micromirrors of the deflected DMD in a strip form to reflection state, acquiring reflected light intensity of a corresponding position by the single photoelectric detector to obtain a column vector, namely projected light intensity of diffracted light; finally, calculating particle size distribution by using Abel inverse transform and integral inversion. The particle size measurer is simple in structure, convenient to operate and high in calculating speed, and has high application value and promising application prospect in the field of particle size measurement.