To achieve high-precision measurements of salinity, temperature, and pressure, a Mach-Zehnder interferometer（TMZI）based fiber-optic conductivity-temperature-depth（CTD）sensor enhanced by the virtual vernier effect is proposed. By optimizing the fiber structure, the sensor overcomes the mechanical fragility issues associated with traditional single-mode fibers and micro/nano fiber couplers. The spectral data is processed using fast Fourier transform（FFT）and inverse fast Fourier transform（IFFT）, and combined with the virtual vernier effect, significantly improving measurement sensitivity. The experimental results indicate that the sensitivity reaches 37.310 nm/‰ and 16.361 nm/‰ within the salinity ranges of 0~5.85‰ and 5.85‰~19.55‰, respectively, the sensitivity of temperature measurement is-27.667 nm/°C within the range of 30.86~41.95℃, and the sensitivity of pressure sensing is 51.087 nm/MPa within the range of 0~4.5 MPa.