A highly sensitive strain sensor is demonstrated by introducing a photonic crystal fiber (PCF) Mach-Zehnder interferometer (MZI) in a cavity ringdown fiber loop as a sensing element. The MZI is fabricated by splicing a short length of PCF between two single-mode fibers with collapsed air holes over a short region at two splicing points, which allows coupling between core and cladding modes inside the PCF. By measuring the decay constants of the fiber ringdown loop under different applied strains, a high strain sensitivity of ~0.21 μs⁻¹/εm and a minimum detectable strain of ~3.6 με are obtained. As a benefit from the ultralow thermal dependence of PCF, the maximum temperature-induced measurement error could be reduced to ~0.24 με.