Temperature distribution in the bio-tissue during PDT is theoretical analyzed based on the Pennes' bio-heat transfer equations. The study indicated that Laser heat source can be formed in bio-tissue during laser irradiation which result in temperature rising in one-dimensional space. The temperature in biological tissue increases in exponent with time and decreases in exponent with the distance from laser irradiation area. There is minimal light dosage threshold during the optical-thermal interaction between laser and biological tissue. When light dosage below the critical threshold,laser irradiating will not produce thermal effect. The threshold,which depends on optical parameter and heat parameter of biotissue,increases in exponent with the distance from laser irradiating area.