In this paper, a novel strain sensor based on composite interference established within an S-tapered multimode (STM) fiber structure is proposed and experimentally demonstrated. The STM fibre structure is simply realized by non-axially tapering a traditional single-mode-multimode-single-mode (SMS) fiber into S-shape using a fusion splicer. This fabricated S-tapered structure provides an extra Mach-Zehnder interferometer (MZI) that is introduced within the multimode fibre (MMF) section; therefore, composite interference based on the inherent multimode interference (MMI) of an SMS and the introduced MZI is successfully established. This resultant composite interference greatly enhances the performance of traditional SMS fibre structures for strain sensing, with a maximum strain measurement sensitivity as high as -103.8 pm/με achieved with a detectable strain resolution of 0.2 με. Benefiting from the experimentally determined high sensitivity and good repeatability, this low-cost strain sensor can be realistically applied in many areas where high accuracy strain measurement is required.