Photoionization spectrum measurement method was designed based on constant photocurrent control by PID technology. Combined with photocurrent and hall effect measurements, this method can provide exact photoionization cross section in GaN epilayers. The measurement results of GaN epilayers show that, the responsiveness of photoelectric detector is the dominating factor affecting test accuracy. The test error increases with the incident photon energy. An 8% test error can be produced under incident photon with 3.2 eV photon energy. Deep level trap in GaN epilayers can still absorb photons with incident energy less than the energy difference between deep level trap and conductor band(2.85 eV), which implies that the lattice relaxation associated with deep level trap takes places in GaN epilayers.