The essential problem in producing two-layer non-quaterwave laser anti-reflection coating with minimum reflectance is pointed out in the paper. The difficulty is that the thicknesses of the lowest layers on different substrates can t be well controlled with the game accuracy because of the difference of surface states in substrate. It is demonstrated with vector diagram method that the important way to solve the difficulty mentioned above is to add a layer so as to compose three-layer anti-reflection coating, of which the lowest layer has refractive index as approximate to the index of the substrate asi possible.