The diffusion equation describing the light transportation in two dimensional two layers dental model with finite size is solved by finite element method,and the optical intensity distribution inside the dental tissueis is obtained.The penetration depth is defined and analyzed when dental enamel and dentin is under different optical parameters.It is revealed that penetration depth decreases with the increase of scattering coefficient of dental enamel,but increases with the increase of scattering coefficient of dental dentin.In the case of variation Rate of penetration depth to scattering coefficient,dental enamel is much larger than dentin.And,the Monte Carlo random statistical method is used to verify the correct of solution of diffusion equation based on finite element method.