A model is built when atoms in the radiation of monochromatic light. By means of the perturbation method,the probability is obtained for atoms in two energy levels. Long-time action of atoms in the radiation of monochromatic light is discussed. On the condition of the speed of atoms on the influence of radioactive field is more than the decay speed of energy level,atoms oscillate between two energy levels. Atoms and radioactive field interact. Stronger,their frequency district is wider. It is concluded that the transition speed of different energy levels does not change with the time when atoms in the radiation of monochromatic light long time.