The strong evanescent field of micro/nanofiber enables it to have a stronger interaction with the outside world and to better sense the changes of the external environment, thus realizing many types of chemical, biological, and environmental sensors. We applied micro/nano-optics for ocean exploration and proposed a sensing probe based on an elliptical microfiber. The sensor probe utilizes the stability of polarization intermode interference of an elliptical fiber and the high sensitivity of the microfiber evanescent wave coupling to achieve a high accuracy measurement of seawater salinity. Laboratory salinity sensitivity of 45.6 nm/% was achieved under seawater environment using an elliptical microfiber with a diameter of 2.8  μm and an ellipticity of 0.3, which is two or three orders of magnitude higher than that of conventional salinity measurements. This sensing probe is used to measure the salinity of seawater in the coastal area of Qinhuangdao, samples were taken for 7 days in September, and the average salinity of the seawater was measured to be 2.956%, which is basically consistent with the data in the Marine Environmental Impact Assessment Report Form of Qinhuangdao Government Service Center.