This paper aims to solve these difficulties: complexities or maloperations of line protection device, the influence on protection performance during transmission line bypass operation. A new scheme of line protection with bypass line protection is introduced. This scheme is applicable to smart substations. The overall plan and criteria of the new scheme, as well as implementation in smart substation are discussed in detail. In this scheme, a bypass line protection device can be spared, operations of line protection device during transmission line bypass operation can be simplified. Moreover, the influence on protection caused by paralleling branch current can be eliminated, and security of power system can be enhanced.