A multi-scale model linking microstructure and macro electric properties of ferroelectric field effect transistor (FeFET) is established based on the phase field method and Maxwell's total current law. The simulation results confirm that the channel current of FeFET is modulated by local interface ferroelectric domains which are easily affected by the microstructure. Using the developed transistor model, we investigate the effects of location of dislocation sites on hysteresis behavior of ferroelectric film and electric behavior of FeFET. Interfacial dislocations have strong effects on hysteresis behavior than internal dislocations since the former can induce imprint behavior while the latter only reduce the coercive field. However, due to the c-domains at the interface, the effects of interfacial and internal dislocations on FeFET electrical properties are the exact opposite. The interfacial dislocations just increase the voltage required to toggle the channel while the internal dislocations can induce disappearance of the FeFET memory window.