The full orbit of energetic ion in tokamak is simulated by strictly solving Hamiltonian equations in guiding center coordinate system, and the semi-trapped orbit which is predicted in a recent theoretical research and the stagnation orbit are confirmed by the presently developed full orbit theory. The prompt loss of the energetic particle in international thermonuclear experimental reactor is calculated according to the present full orbit theory. It is found that the prompt loss rate of energetic ion in the full orbit theory can be larger than 14% the result in gyro-averaged theory, and the detailed analysis is carried out and physical understanding is presented in this paper.