A sheath type disposable safety scalpel consists of the blade, handle and sheath, and when the sheath slides forward and backward in relation to the handle, the blade shows the extension and retraction states in the sheath, wherein: the positioning mechanism consists of the button on the sheath and the blade protection locking notch (7) and blade exposure locking notch (8) on the handle, and the unlocking inclined surface is set under the blade protection locking notch and blade protection locking notch, and when the button is pressed and pulled, the locking head touches the unlocking inclined surface and climbs out of the locking notch along the unlocking inclined surface to unlock, so that sheath slides between blade protection locking notch and blade exposure locking notch in relation to the handle. The present invention breaks the monopolization of traditional positioning mechanism, improves the safety of scalpel and improves the user hand feeling and sound experience.
