The present work is mainly to study two-photon process in H2X (X=O, S, Se, Te) by using the full relativistic theory. For comparison, we also study the single-photon process by SAC-CI method. The transition probability of two-photon excitation is 10-2–10-5 times of the single-photon process; the relativistic effects become more and more obvious with the increase of atomic number. In addition, every molecule observes the selection principles; dipole transition component and oscillator strength of individual symmetrical states are greater than those of other individual states. This is due to the symmetry of molecule and it should be an important basis for selecting transition energy.