Finite element analysis is used to study the near infrared light distribution in brain tissue. The diameter of the light beam varies from 100 microns to 1 mm. The researching results indicate that the near infrared light distribution in brain tissue gets wider with rising of the beam diameter, and the distribution fields are 0.05-0.2 mm below the probe, respectively. At last, the results of finite element analysis are verified by Monte-Carlo simulation, which prove the feasibility of using near infrared light in brain surgical guidance.