An in-mold labeling process attaches a planar flexible label to a three-dimensional curved surface of a container. The planar label is punched from a planar flexible base material according to a preset shape, which is a planar shape formed by unfolding the three-dimensional face of the container. During molding, air pressure keeps the planar label at a pre-labeling position and deforms the label along an inner wall of a cavity to maintain adhesion, so that the planar label is converted into a three-dimensional label. Based on the characteristics of the flexible material, the label deforms along the inner wall of the cavity while maintaining the outer contour of the label flush with the periphery of the pre-labeling position. The process allows the label and container to be integrally molded, ensuring the stability of adhesion and providing better wear resistance and protection.