A multimode-fiber Mach-Zehnder interference strain sensor with threaded grooves is proposed. The threaded groove structure is made on the multi-mode fiber as the sensing area, and the cross-sectional area of the spiral groove fiber is smaller than that of the single-mode fiber, so that the actual strain on the sensing area is greater than the applied strain. When a quantitative strain is applied to the fiber, the wavelength drift is greater, which increases the strain sensitivity of the sensor. The experimental results show that in the strain range of 0~900 με, the strain sensitivities of the single- and multi-mode fiber sensors are -6.44 to -16.89 pm/με, respectively, with linearities of 0.979 and 0.981, respectively. The sensor has the advantages of simple structure, high sensitivity, and high mechanical strength, providing a new solution for enhancing the sensitivity of optical fiber interference strain sensors.