Abstract: Objective:To examine the effects of photodynamic therapy(PDT) on pseudomonas aeruginos strain in vitro. Methods:The pseudomonas aeruginos strain is used to investigate the photodynamic efficacy of PDT, methylene blue (MB) be the photosensitizer and laser be the light source (maxoutput=300 mW). The cultured bacterium is treated with different concentrations of MB under different laser irradiation dosages. The bacteria groups were counted to measure the relative bacteria inhibitory rate introduced by PDT. In addition, the rabbit blood agar is used to detect the changes of nosogenesis to pseudomonas aeruginos strain.　Results:With same light dose condition, the photodynamic efficacy and a reduced nosogenesis on pseudomonas aeruginos strain are most notable given proper concentration (131.7 (mol) methylene blue solution, while higher (1317 (mol) or lower (13.17 (mol) concentration level does not present obvious results.　Conclusions:PDT has significant photodynamic efficacy on Pseudomonas aeruginos strain in vitro, its relative inhibitory rate presents close correlation with the dose of sensitizer and light irradiation. Hence discovering proper photosensitizer concentration during therapy is highly important.