Polyamic acid(PAA) solution based on 4,4'-diaminodiphenyl ether(ODA) and pyromelliticdianhydride( PMDA) was prepared and electrospun into PAA nanofiber films. In-situ Fourier transform infrared (FTIR) spectroscopy was used to study the progress of imidization. Through applying the optimized condition of imidization, a polyimide film was obtained with 100% of imidization degree. The results show that lower heating rate (2 degrees C/min) leads to a 100% of imidization degree; PAA nanofiber films exhibited lower imidization degree under the faster rate. A new fast-slow heating rate method can improve the imidization efficiency.