A collapsible highchair with a locking device includes a first frame, a second frame which is pivotally connected to the first frame in “X” shape, a seat portion pivotally connected to upper sections of the frames, a backrest connected to the upper section of the first frame, a locking device secured to top end of upper section of the second frame, and an armrest one end of which is pivotally connected to the first frame and another end is engaged with the locking device. The locking device includes a first case and a second case, both of which cover the second frame. The first case is integrally formed with an elastic actuating portion. When the actuating portion is engaged with an opening on the armrest, the highchair is in a stretched state. When the actuating portion is pressed to disengage from the opening, the highchair is in a collapsible state.