The near infrared technique is used for 3D accurate location of the probe in neuro-surgery of Parkinson's disease. The optical probe's Look Ahead Distance(LAD) can be used to guide the probe. By the simulation of Monte Carlo and research of the 2－layer laboratory phantoms,the LAD's formulations are given. With the comparison of LADs from both Monte Carlo and fitting equation,it can be proved that the formulations are correctly . Using LAD can improve the precision of the location and shorten the time of operation. Analyse the 3－layer model and give the definition of Layer Resolution(LR). At last,use the LR to locate the optical probe in the animal test.