In order to achieve the purpose of measuring the height change of the liquid, the method based on the multimode interference effect of the hollow core fiber was used to study the change of the interference spectrum caused by the multimode interference of the light source in the hollow core fiber under the influence of the external medium. The liquid level sensing experiment of the multimode interference effect in the core optical fiber was carried out. The relationship between the interference spectrum of the liquid level sensor and the change of the liquid level and the influence of liquids with different refractive indexes on the measurement results were experimentally studied, and the experimental errors were analyzed. The results show that the liquid level measurement range of the fiber optic liquid level sensor is 0mm~55mm, when the liquid refractive index is 1.33 and 1.35, the liquid level measurement sensitivity is 0.180nm/mm and 0.224nm/mm, respectively. It is more accurate and feasible to use a sensor with a single-mode-air-core-single-mode structure to measure liquid level changes.