It is desirable for the non-and-mini traumatic laser surgical procedures to remove tissue accurately with the minimal thermal damage to the adjacent non-irradiated tissues. Photo-thermal interactions between laser rays and biological tissues have various thermal effects such as coagulation, vaporization and carbonization. The energy of laser continuously accumulates and diffuses in biological tissues, so that different extent thermal effects and thermal damage of biological tissues have been induced. In this paper, the thermal conduction model and optical parameters of biological tissues are discussed, the relevant thermal effects calculation equations are deduced, the instantaneous temperature distribution of biological tissue irradiated by high power laser is simulated, the thermal damage depth is numerically predicted and the relationship curve between time and the thermal penetraction depth is described. The study is of significance in revealing the mechanisms of laser thermal interaction with biological tissue, as well as developing clinical application of laser medicine.