The invention relates to a particle size distribution measurement apparatus and method based on a closed-loop control principle. The apparatus comprises elements including a visible laser, a beam expanding lens, a diaphragm, a sample cell, a Fourier expansion lens, an annulus photoelectric detector, a data collection card and a host computer. The method comprises: firstly, obtaining small-angle diffracted light distribution through a diffraction-type measurement apparatus; then, representing a k-time reconstruction result of particle size distribution with f(x), and obtaining initial value of f<0>(x) and f<1>(x); then, based on a closed-loop control principle, taking a reconstruction error as a control object, and using an iterative formula to reconstruct particle size distribution; and finally, judging whether an iteration termination condition is met, if so, stopping iteration and outputting a reconstruction result, otherwise returning to the iterative formula in the previous step. According to the apparatus and method, the affect that diffracted light information obtained in the practical application is limited is prevented, and the resolution capability and reconstruction precision of particle size distribution are improved. The apparatus and method have significant practical value and bright application prospects in the field of particle size distribution measurement.