This paper introduces configuration of interlock logic of YINCHUAN-DONG converter station breaker in the middle in YINCHUANDONG-JIAODONG ±660 kV HVDC project, analyses the current interlock logic of YINCHUAN-DONG converter station breaker in the middle, and proposes the problems and consequence of the current interlock logic in operation. According to the problems of current interlock logic in operation, this paper proposes a new interlock logic of breaker in the middle based on the line-transformer strings with 3/2 connection breaker protection. The new interlock logic eliminates throughly dead zone of current logic, adapts to all kinds of operating conditions, and improves effectively the reliability of HVDC, which can avoid the serious accident of the AC system and HVDC system blocking because of the interlock logic of breaker in the middle not meeting conditions.