Abstract: A retractable knife has a blade cartridge removably received in a cavity of handle. The cartridge comprises two clips on top and bottom sides, an elongate top slot, a rear slit two elastic members having top ends urged upward against a bottommost backup blade, a limiting protrusion engaged with a topmost backup blade for preventing it from moving out of rear slit incidentally, and a guide rail engaged with a second topmost backup blade for orderly guiding backup blades toward slot. In an operation, push a push button rearward to move the topmost backup blade out of the slit. Further, in another operation, push clips rearward for removing blade cartridge for replacement.