The invention provides a limb protection air bag in a vehicle cab, and belongs to the technical field of vehicles. The limb protection air bag solves problems that an existing air bag cannot protect limb at the same time and impact force is large at the opening moment. The limb protection air bag in a vehicle cab comprises a housing, a cover plate, an air pocket, and a gas generator. The air pocket comprises an upper part which expands upward after inflation, and a lower part which expands downward after inflation. The upper part and the lower part respectively comprises a front sheet which faces the side of a seat after expansion, and a rear sheet connected with the gas generator. Unelastic draw strings are respectively connected between the gas generator and the front sheet of the upper part, and between the gas generator and the front sheet of the lower part. The rear sheets of the upper part and the lower part are respectively provided with a plurality of folds. The openings of the cover plate and the housing are provided with connection structures which make the cover plate move a distance relative to the housing when the air pocket is inflating. A U-shaped large volume air pocket makes the cab has a function of protecting limbs, and arrangement of the draw strings, the cover plate, stay wires, and a spacing belt makes the air bag smoothly unfold, and the air bag is safer when being unfolded.