In order to measure the particle size in the medium using near-infrared light source, a particle size measurement method based on light extinction method combined with database technology is proposed. The extinction spectrum database related to different particle size and distribution is first calculated and generated by the total scattering extinction Lambert-Beer law. The database processing algorithm is employed to compare the data in the database with the measured particle extinction spectrum data. Then the particle size and distribution parameters of the measured samples are determined from the calculated values of the best match in the database. Next, numerical simulations of both the unimodal and bimodal particle size distribution are carried out, and the relative root mean square error of the volume frequency distribution of the simulation results is less than 10%. Three kinds of national standard particles with different particle sizes are used for experimental verification, and the relative errors of the measured particle size D50 are all less than 10%. Thus the validity of the proposed method and algorithm is demonstrated through simulations and experiments with satisfactory results. The research in this paper shows that the application of database technology and algorithm to the measurement of particle size by near-infrared narrow-band extinction method is feasible and effective.