Making use of a set of quantum chemistry methods, the harmonic potential surfaces of the ground state (S0(1 A g)) and the first (S1(1 B 3u)) excited state of pyrazine are investigated, and the electronic structures of the two states are characterized. In the present study, the conventional quantum mechanical method, taking account of the Born-Oppenheimer adiabatic approximation, is adopted to simulate the absorption spectrum of S1(1 B 3u) state of pyrazine. The assignment of main vibronic transitions is made for S1(1 B 3u) state. It is found that the spectral profile is mainly described by the Franck-Condon progression of totally symmetric mode ν6a. For the five totally symmetric modes, the present calculations show that the frequency differences between the ground and the S1(1 B 3u) state are small. Therefore the displaced harmonic oscillator approximation along with Franck-Condon transition is used to simulate S1(1 B 3u) absorption spectra. The distortion effect due to the so-called quadratic coupling is demonstrated to be unimportant for the absorption spectrum, except the coupling mode ν10a. The calculated S1(1 B 3u) absorption spectrum is in reasonable agreement with the experimental spectra.