The invention discloses a monitoring method for simulating the annulus displacement efficiency of shale gas horizontal well cementation. The monitoring method is realized through a low-field nuclear magnetic resonance imager and comprises the steps of 1, placing an annular measurement pipe section into a radio-frequency coil and injecting displacing liquid into a shaft; 2, adjusting the parameters of the low-field nuclear magnetic resonance imager and enabling the measurement pipe section sample to generate nuclear magnetic resonance; 3, sending nuclear magnetic resonance signals of the measurement pipe section sample to a radio frequency unit and converting the signals through a spectrometer system for obtaining spectral lines of the nuclear magnetic resonance signals; 4, obtaining imaging pictures of the measurement pipe section sample in different directions; 5, injecting displacing liquid into an inner pipe of the shaft, repeating the steps 2-4, obtaining T2 spectral lines and imaging pictures of the measurement pipe section sample at different moments, drawing a curve chart of the displacement efficiency and conducting research on the mixing and displacing mechanism of the fluid through the imaging pictures. According to the monitoring method, the flow condition of annulus liquid can be effectively monitored, the mixing and displacing mechanism of the fluid can be researched through the imaging pictures, and the shortcomings of the prior art can be overcome.