Transmission properties of the ray in laser intravascular irradiation blood therapy are researched with Monte Carlo method.The distribution and absorption of photons are obtained in different conditions,for example,in different incidence and emanation angles,in different blood vessels or different incidence points which deviate from center axis of the blood vessel.According to our research,the incidence fiber must parallelism with the blood vessel,the angle between the fiber and the vessel must be small and the diameter of the blood vessel must be larger than 6 mm in laser intravascular irradiation blood therapy.Only in this way,the incidence laser can be fully absorbed and the influence caused by the incidence point deviation can be reduced.