Optical second-harmonic generation technology is an important tool for studying the polarization characteristics and symmetry of materials. Second-harmonic generation test is carried out on BaTiO₃ film samples grown on SrTiO₃ substrates by using the built-up microscopic second-harmonic generation light path. The results show that BaTiO₃ film has double rotational symmetry in the in-plane direction. The second-harmonic signal of the sample was measured in four polarization modes. The results show that with the change of the incident power of the fundamental frequency light, the second-harmonic intensity pattern also changed. When the incident light power is 7.5--7.7 mW, the signal changes suddenly; when the incident light power is greater than 7.5 mW, the main peak of the second-harmonic pattern of the horizontally polarized outgoing light pattern is weakened, and the secondary peak is strengthened.