A non-intrusive optical technique for the study of moving particles and their statistics, laser phase Doppler anemometry (PDA), is used in on-line inspecting and quantifying boiler dusty. PDA collects and processes Doppler signals from moving scattering particles and provides a deterministic measure of particle velocity and statistical information about particles’ size and size distribution. Furthermore, PDA is superior to the traditional intensity based optical particle analyzer, as Doppler frequency or phase shift is less susceptible to electronic noise than intensity. These features are important for reliable results, especially at in-situ application. The present system operated successfully in inspecting soot letting of coal-fired boiler, which leads to an active structure that can be applied in the conditions of limited optical access. The measuring results of soot parameters, as well as the statistics of soot are presented in this paper.