Extinguishing concentration measurement experiments in the cup burner on two kinds of superfine ammonium phosphate extinguishing agents with different particle sizes had been done by means of filtering and weighing method combined with laser attenuation measurement. The fitting curves of particle concentration with laser attenuation rate and the extinguishing concentration of superfine particle agents had been got. The extinguishing mass concentration of two super fine agents were measured in cup burner. The results showed that when the median particle size of two kinds of extinguishing agents were 6.0 μm and 13.7μm, their laser absorption coefficients were 0.353 m²/g and 0.257 3 m²/g respectively and their mean extinguishing concentration were 32.9 g/m³ and 41.6 g/m³ respectively. Extinguishing ability of the former was 25 % to 30 % higher than that of the latter. The laser attenuation measurement relative errors were less when the former agent was used in experiment.