In this paper the exciton binding energy in wurtzite GaN quantum dot is investigated by considering the influences of polarization in details. We find that the strong built-in electric field gives rise to an obvious modification of conduction band profile of QDs and leads to remarkable electron-hole spatial separation. This effect has a significant influence on exciton states and optical properties of the QDs. The relationship between exciton states and QD size and QD confinement potential is studied. Our results represent the decrease of exciton binding energy by considering piezoelectric polarization.