The dense fluid helium will be ionized under high pressures and temperatures. The ionization energy of helium will be lowered as the result of interactions between all particles of He, He+, He2+, and e. In this paper, the thermodynamic parameters are obtained from nonideal ionization equilibrium, taking into account the correlation contributions to the chemical potential which is determined self-consistently by the free energy function. The theoretical equation of state (EOS) is verified by shock compression experiments and other theoretical calculations. The EOS of dense helium plasma are predicted and the contributrions of interactions are discussed for the density and temperature range of 10-3—100.3 g/cm3 and 4—7 eV, respectively. The calculations show that the pressure ionization is caused by the reduced ionization energy induced by the nonideal interactions, and the ionization energy should be corrected at high temperatures and densities.