Amorphous SmCo thin films with thickness of 10–150 nm were deposited on the flexible polyethylene terephthalate (PET) substrates. Tensile/compressive strain was generated in the amorphous SmCo thin film when PET substrate was flattened from concave/convex shape after thin film deposition. Results show that both the normalized remanent magnetization and the squareness of hysteresis loops of SmCo/PET can be tuned by the strain. Compared with that induced by compressive strain, the tunable amplitude induced by tensile strain is larger for the amorphous SmCo thin films. Because the amorphous SmCo thin film has negative magnetostrictive property, the magnetic properties of amorphous SmCo thin film can be controlled by the mechanical strain supplied by flexible substrates. When the negative magnetostriction effect occurs, the magnetization process of amorphous SmCo thin films is hindered by the tensile strain, whereas it is promoted by the compressive strain. The amorphous SmCo/PET shows great potential in the field of flexible spintronic devices and flexible micro-nano electronic devices.