A carpet knife having a housing, an active blade installed in the housing, and a blade clamping means having a clamping cover and a thumb screw. The housing has a thumb screw hole, a pin hole, at least one side guiding groove, and an active blade installation bar. The clamping cover has at least one side guiding bar having a wedge outer surface, and a rear eccentric guiding groove. The thumb screw has a head having an under surface and a shank formed on the under surface. The head has a pin formed on the under surface thereof which is able to enter and move in the rear eccentric guiding groove.