Strain gradient is a normal phenomenon around a heterostructural interface in ultrathin film, and it is important to determine its effect on magnetic interactions to understand interfacial coupling. In this work, ultrathin Pr_0.67Sr_0.33MnO₃ (PSMO) films on different substrates are studied. For PSMO film under different in-plane strain conditions, the saturated magnetization and Curie temperature can be qualitatively explained by double-exchange interaction and the Jahn-Teller distortion. However, the difference in the saturated magnetization with zero field cooling and 5 T field cooling is proportional to the strain gradient. Strain-gradient-induced structural disorder is proposed to enhance phonon-electron antiferromagnetic interactions and the corresponding antiferromagnetic-to-ferromagnetic phase transition via a strong magnetic field during the field cooling process. A non-monotonous structural transition of the MnO₆ octahedral rotation can enlarge the strain gradient in PSMO film on a SrTiO₃ substrate. This work demonstrates the existence of the flexomagnetic effect in ultrathin manganite film, which should be applicable to other complex oxide systems.