Safe and efficient demolition of reinforced concrete support beam in deep foundation pit under complex environment is a key point in the process of urban construction. On the basis of theoretical analysis on lateral restraint mechanism of stirrups, the evolution law for the stress state of the concrete element and the blasting effect of the supporting beam is revealed before and after the remove of the lateral restraint effect of the stirrup. The methods for stirrup cutting, shothole arrangement and millisecond delay initiation network are developed. A comparatively systematic technology on blasting demolition of support beams is formed. The result of engineering application shows that, the blasting demolition effect can be significantly enhanced by the linear cutting pretreatment method. The coupling state of explosion energy can be improved and the hazard of blasting flying rocks can be reduced by the method of multi-directional synergy arrangement of shotholes. The vibration intensity can be effectively controlled and the network reliability can be improved by the millisecond delay detonating network based on the blasting network connection unit.