Density functional theory has been applied to in-depth study on photoelectron spectrum of BCl+3 cation in the C2?A''2 state. With the aug-cc-pvtz basis set, the geometries of neutral BCl3, BCl+3 cation in ground state, X2?A2', and an electronically excited state, C2?A''2, are optimized using three theory levels of PBE0, ωB97XD and M06-2X. Then the corresponding vibrational frequencies and the Franck-Condon factors of the BCl3(X1A1)→ BCl+3(C2?A''2) transition are calculated, and the photoelectron spectrum is simulated. By comparing with the high-resolution photoelectron spectrum obtained previously in experiment, the photoelectron spectrum of the C2?A''2 band is successfully simulated, then reliable assignments for the observed vibrational progression are obtained. Based on these vibrational assignments, the adiabatic ionization energy (AIE) and vertical ionization energy (VIE) for BCl+3 cation in the C2?A''2 state have been determined directly as AIE(C2?A''2)=(14.298 ±0.028) eV and VIE(C2?A''2)=(14.405 ±0.028) eV, respectively, and their wrong values reported in previous experiments have been corrected.