Thermally activated delayed fluorescence (TADF) emitters, with internal quantum efficiency (IQE) in organic light-emitting diodes (OLED) approaching 100%, have attracted great attention recently. However, theoretical investigation on the electroluminescent mechanism of TADF emitters is quite rare. In this paper, the time-dependent density functional theory is used to study the property of excited states of the TADF emitters, and it is found that both the geometry and the electronic structure are quite dependent on the functionals. By comparing with the experimental results, a ‘hybrid’ method is adopted to study the photophysical properties of the TADF emitters. Based on the energy structure of the states, the lowest three states are found to have close relation to the electroluminescent process. The dynamics of two lowest excited states are investigated and the rate equation is used to analyze the evolution of the three states involved. A visual picture of the exciton evolution process is obtained, and one can get a better understanding of the up-conversion mechanism of TADF emitters. The analysis of the electron distribution of the transition orbitals indicates that the first singlet excited state of the molecule possesses both the charge transfer and local excitation components, which is a necessary character for a TADF emitter. The comparison of the property of two isomers indicates that the appropriate arrangement of donor groups and acceptor groups is important for a high-efficient TADF emitter.