Abstract: The method to control the width of crack at bottom of reinforced concrete slabs by adjusting plastic moment ratio of slab bottom is proposed, and the plastic-moment-ratio formula is established to enhance the character of anti-crack at bottom of reinforced concrete slabs based on Navier’s thin slab theory. The plastic-bending-moment ratio formula which guarantees the least amount of reinforcement at slab bottom is also obtained by means of plastic hinge theory. Through comparison, the proposed formulas pertinent to bending moment ratio at slab bottom applied in practical engineering are suggested as well.