Ultrasonic detection methods are vital in industrial and medical applications due to their non-radiative, sensitive, and high-resolution properties. Here, we propose a novel compact fiber-optic ultrasonic sensor based on multicore fiber. This sensor exploits the evanescent field effect in a tapered MCF region, enabling sensitive inter-mode interference and intrinsic immunity to electromagnetic interference. The sensor’s structure consists of a cascaded single-mode fiber (SMF)-MCF-SMF configuration, where the MCF is tapered to enhance sensitivity. Theoretical analysis based on coupled-mode theory elucidates the sensor’s operation, while the fabrication process ensures all-fiber integration and miniaturization. Experimental results demonstrate the sensor’s capability to effectively detect ultrasonic signals up to 2.5 MHz, highlighting its potential for high-frequency ultrasonic sensing in power equipment.