In this talk, I will give a brief introduction to the multi-configuration Dirac-Hartree-Fock method and the active space approach for an relativistic many-electron atomic system. Then I will present some of my works that we have done or that we are doing. These works are mainly about accurate calculations of lifetimes of atomic states, isotope shifts, and magnetic field effects in atoms. The emphasis will be put on the applications of these results either to diagnosis of plasma states or to extractions of nuclear parameters such as nuclear charge distributions.