Density functional theory (DFT) and time-dependent DFT calculations were performed to evaluate the influence of substituent effect of (1) R = 4-Me, (2) R = 4-OMe, and (3) R = 2,3-OC6H4 on the phenyl ring of (C∧C*)PtII(acac) (C∧C* = phenylimidazole, acac = acetylacetone), respectively, on absorption and phosphorescent spectra properties, as well as the radiative and nonradiative processes. We found that emissions of complexes 2 and 3 originate from the Kasha-like T1 state, whereas that of complex 1 originates from non-Kasha T2 state. Compared with the emission of complex 1, the emission peaks of 2 and 3 are red-shifted, which is attributed to p-π and π-π conjugation effects resulting from the electron-donating groups -OCH3 and -OC6H4 with ligand C∧C*, respectively. The radiative rate constants (κr) of 2 and 3 are larger than that of 1, namely, κr(1) < κr(2) < κr(3), indicating that κr can be efficiently increased by enlarging π-conjugation at the main ligand of (C∧C*)PtII(acac), which can cause the increase of spin-orbit coupling (SOC) matrix elements. At the same time, the activation energy barriers for the rate-limiting step can be largely raised accompanied by enlarging the ability of electron-donation of the substituent group at the main ligand of (C∧C*)PtII(acac), which can cause the decrease of the nonradiative rate constant (κnr), namely, κnr(1) > κnr(2) > κnr(3). According to ΦP = κr/(κr + κnr), the quantum yields should have the sequence ΦP(1) < ΦP(2) < ΦP(3), which is in accordance with the experiment. In addition, to guide experimental synthesis of highly efficient (C∧C*)PtII(acac), a new complex 4 through extending the π-conjugation in the C∧C* ligand of (C∧C*)PtII(acac) was theoretically designed, which has a larger quantum yield than 1-3.