Abstract: Through establishing mathematical relation model of laser, photo-sensitizer and oxygen, photodynamic dose (PDD) in tissue was simulated and its distribution was studied. The change of PDD under the different laser doses of 50mW/cm2，100mW/cm2 and 150mW/cm2 and the photo-sensitizer concentrations of 10mg/kg，20mg/kg and 40mg/kg were simulated. It is concluded that the PDD, an inhomogeneous value in the tissue, changes with many factors such as laser dose, photo-sensitizer and oxygen, and it is obviously effected by the variation of photo-sensitizer concentration.