In this work, some critical structures (e.g. stable structure, transition state, local minimum and conical intersection) of azobenzene photoisomerization were optimized by means of ab initio CASSCF calculation. The potential energy surfaces for the CNNC dihedral torsion and CNN bond angle concerted-inversion pathway were mapped to explore the relaxation process of azobenzene (AB) photoisomerization. The results indicate that the rotational mechanism favors the photoisomerization of the S(1)(n,pi*) and S(2)(pi,pi*) trans-AB. The concerted-inversion mechanism may operate in the decay process of S(2)(pi,pi*) or higher state trans-AB. By borrowing the (n,pi*; pi,pi*) and (n(2),pi*(2)) electronic states, trans-AB upon excitation to the higher states can quickly relax to the S(1)(n,pi*) or ground state via the rotation or concerted-inversion pathway. The forming ground-state species with higher vibrational energy from the higher excited states will become the stable trans-isomer through the concerted-inversion pathway. These relaxation processes have been confirmed by the conical intersections calculated by the high-level CASSCF method.