The invention relates to a clutch device of a lap evenness instrument, and relates to the technical field of cotton textile machinery. A driving motor on the lower part of a rack is connected with a lower cotton outlet roller through a chain. The lower cotton outlet roller and an upper cotton outlet roller are in mesh transmission. A lap roller is arranged on the upper part of the rack. A cotton pressing roller is connected with the lap roller. The lower part of the lap roller is connected with a clutch gear through gear transmission. A swinging bracket is connected with an expansion device push rod. The clutch gear is meshed with a drive gear of the lower cotton outlet roller. An optoelectronic switch is arranged on the upper part of the rack. A drive gear of the lap roller is provided with a switch probe. The optoelectronic switch contacts with the switch probe. A lap is arranged on the upper part of the lap roller. A tray bearing a cotton layer is arranged below the lower cotton outlet roller. The tray is connected with the rack. One end of the optoelectronic switch is connected with a power line through a time relay, and the other end is connected with the power line through afuse. The device solves problems of component wear and measured deviation, eliminates potential production quality hazard, and is simple in structure and safe in operation. The device effectively saves machine materials, and ensures normal operation of production.