The Landau distribution is an important distribution in physical sciences that describes the statistical property of the energy loss by charged particles as they traversing a thin layer of matter. However, this distribution is difficult to treat analytically and requires integration for numerical evaluation. The CERNLIB-G110 approximation, which is currently widely used, is accurate to within 8-9 digits of the relative error. In this paper, an approximate formula with 16-digit relative error, which is accurate enough for double type of IEEE754, and an approximate formula with 34-digit relative error for float128 are obtained. In addition to the approximation, numerical evaluation of Landau distribution and random number generation method are also presented.