Poly(amic acid-co-imide) (PA-I) is an intermediate for preparation of polyimide from polyamic acid (PAA). Phase separation does not appear for the solution until a certain imidization degree. The experimental results of a rotation viscometer and FT-IR analysis showed that the critical point of phase separation (CPPS) was related to the imidization methods (thermal or chemical imidization) and the backbone structures (flexible ODPA-ODA or semi-rigid PMDA-ODA). Phase separation time is shorted with the increase of the initial PAA concentration, acetic anhydride amount, or temperature. When phase separation occurs, the solution viscosity increases, while the imidization degree is almost constant as the initial PAA concentration rises. The greater the chain mobility, the higher the imidization degree at phase separation point. At CPPS, higher imidization degree and lower solution viscosity are obtained in the thermal imidization than in the chemical imidization; ODPA-ODA system exhibits higher imidization degree and better solubility than that of PMDA-ODA system.