A clamping device includes a support base, a handle pivoted to the support base, a clamping unit including an upper jaw, a lower jaw and a slide respectively mounted in the support base, and a locking unit including a stop block and a locking bar respectively mounted in the support base. When the handle is pressed, the locking bar is driven to unlock the stop block for allowing movement of the slide, and then the slide is pushed by the handle to lift the lower jaw, causing the upper jaw and the lower jar block to clamp a side rail of an operating bed. When the handle is returned, the stop block is locked by the locking bar, so that the slide is positioned to prevent accidental loosening.