A mag-optical modulator based on periodic parity-time (PT) symmetry structure is proposed, which consists of hydro-based MnFe₂O₄ magnetofluid layer in the middle and periodic PT symmetry units on both sides. The high extinction ratio modulation with gain is realized by using the magneto-optic effect of the magnetic fluid. The transfer matrix method is used to simulate and analyze the structure, it turns out that, for the incident light wave whose wavelength is in the low transmission region of the structure gap band(in the wavelength range from 1513 nm to 1587 nm with 1550 nm as the center), the maximum gain of the modulator for incident light is close to 25 dB, the maximum and the minimum extinction ratio are close to 60 dB and 30 dB, respectively. Simultaneously, the average modulation sensitivity of incident light wave transmission and wavelength shift can reach the maximum of 74.51 dB and 108.2 nm, respectively.