The invention discloses an online transmission multi-core optical fiber SPR sensor. The sensor comprises a quartz tube, wherein multiple single-core optical fibers for input, a multi-core optical fiber, multiple single-core optical fibers for output and multi-core optical fiber SPR sensing elements are arranged in the quartz tube, and the number of the single-core optical fibers for input and output is the same as the number of fiber cores of the multi-core optical fiber; multi-core optical fiber coupling elements are arranged at one end of each of the multiple single-core optical fibers for input and output and the two ends of the multi-core optical fiber respectively, and the multi-core optical fiber coupling elements are connected in a waveguide directional coupling mode to form a connection region of the single-core optical fibers and the multi-core optical fiber; and a coating of the middle section of the multi-core optical fiber is removed, exposed fiber cores are plated with a metal film and different sensitive substances to form an SPR sensing region, and each fiber core forms one independent SPR sensing element. Through the sensor, the SPR sensitive elements can be coatedwith corresponding sensitive substances according to the demand of a user, and the sensor can be applied to online real-time measurement in multiple fields such as biosensing, chemical analysis, drugresearch and development and environment monitoring.