Phase-sensitive optical time-domain reflectometer （Φ-OTDR） requires extremely high sampling frequency because it mostly adopts the coherent detection method. In order to reduce the sampling frequency of the system, a frequency-shifted dual pulse modulation direct detection Φ-OTDR system is proposed. The frequency-shifted dual pulse light with frequency shifts of 79 MHz and 81 MHz, respectively, was generated by the time-delay frequency conversion control of the acousto-optic modulator to modulate the Rayleigh backward scattering signal onto the sidebands with a heterodyne frequency of 2 MHz. And the system sampling frequency is reduced to 10MHz by combining with the heterodyne demodulation technique. The validity of the frequency shift is verified by using the Mach-Zehnder interferometer structure, and the accurate demodulation of the vibration signals of 200Hz, 500Hz, 1000Hz, and 2000Hz is successfully realised at the sampling frequency of 10MHz. In the repeatability experiment, the linear fitting coefficient reaches 0.9989, which verifies the stability of the system for vibration signal demodulation under low sampling frequency conditions, and provides an effective technical solution to reduce the hardware cost and data processing capacity of the Φ-OTDR system.