In this paper, we focus on the neutron irradiation effect on lead zirconate titanate (PZT) piezoelectric thick film with a thickness of 5 μm. The selected fluxes are at relatively low fluxes, including 4 kinds of 0 cm^-2, 10¹¹ cm^-2, 10¹² cm^-2, 10¹³ cm^-2. The key properties of the samples have been systematically characterized, especially the interface defect (energy level, concentration, and capture cross-section) using deep level transient spectroscopy (DLTS). It benefits the throughout understanding of the role of interface defects on the overall properties of PZT a lot. The results showed that neutron irradiation at 10¹¹ cm^-2 increased the activation energy of interface defects in the PZT thick film from 0.9 eV to 1.6 eV. This is a significant factor leading to an 11.2% reduction in polarization intensity, a decrease in dielectric constant, and an increase in leakage current. The degree of impact is positively correlated with the irradiation fluence. The results of this study have important reference significance for the application of piezoelectric materials and devices in irradiated environments.