The present invention discloses a fastener with lateral fastening mechanism, including an outer case, a slide block, a claw-shaped head, and a flexible member. The outer case is a hollow tubular member. A swing lever is pivotally connected inside the outer case. The slide block can move freely to a first position where the slide block is hidden inside the outer case, and to a second position where the slide block sticks out of the outer case. One end of the slide block is pivotally connected to the claw-shaped head, and the flexible member can constantly provide force for pushing the claw-shaped head and the slide block. A guide area of the slide block can interact with the swing end of the swing lever, and in the state of interaction, the position of the slide block can be decided to be either at the first position or the second position.