We present a theoretical study of plasma shielding effects on the energies and oscillator strengths among the levels of the 2s22p2, 2s2p3, and 2p4 configurations of Fe XXI under dense plasma conditions using two kinds of ion sphere potentials combined with the multi-configuration Dirac–Fock model. The relativistic effects, electronic correlation effects, and higher order corrections from the Breit interaction, quantum electrodynamics and the finite nuclear size in the above atomic system are included in the calculations to make the results more precise. The plasma-free values obtained in this work are compared with those from other theories and experiments. Satisfactory agreement is found between them. Systematic trends are observed for all the properties under study with respect to increased temperature and electron density. Our results show that the energy shift of most of the levels becomes more sensitive to the plasma electron density when the plasma electron temperature is lower, while the oscillator strengths of the transition lines vary very little at the range of plasma conditions considered. The present results should be useful for diagnostics as well as for precise spectral modeling in fusion plasmas.