Based on the Markov Chain in Probability Theory and the Bragg-Williams-type equations for equilibrium defect concentrations, the occupancy probability equations and transfer probability of describing antisite defect have been established. At the same time, antisite and vacant site defect concentrations' equations have also been derived. Using the established equations, the transfer probability and the first-principles plane-wave pseudopotential method, the various point defects of NiAl were studied. It is proved from the calculation and the electron structure that the occupancy probability of antisite defect obeys Fermi-Dirac statistics and when the temperature changes from 800 to 1300 K with Ni:Al=1, NiAl anti-site probability is more about 106~109 times than AlNi antisite probability, which proves that NiAl is the dominant point defect. VNi vacant site concentration is more about 105~107 times than AlNi antisite concentration, while NiAl antisite concentration is more about 106~1010 times than VAl vacant site concentration