In the framework of nonperturbative quantum field theory, the critical phenomena of one-dimensionalextended Hubbard model (EHM) at half-filling are discussed from weak to intermediate interactions. After the EHMbeing mapped into two decoupled sine-Gordon models, the ground state phase diagram of the system is derived in anexplicit way. It is confirmed that the coexisting phases appear in different interaction regimes which cannot be foundby conventional theoretical methods. The diagram shows that there are seven different phase regions in the groundstate, which seems not to be the same as previous discussions, especially the boundary between the phase separationand condensed phase regions. The phase transition properties of the model between various phase regions are studied indetail.