Abstract: The approach locking state of a train route has a critical effect on protecting the train after it overrunning the route. The current approach locking processing logic of the train route is based on the design theory of traditional relay interlocking, which can meet the requirement of general scenarios but can not meet that of the scenarios in which the approach locking section will be extended up to or beyond the second route ahead of the train. Besides, this processing logic might lead to the state of the train route wrongly converting to approach locoking in the scenario of special failures. So, this paper puts forward an improved method for extending the approach locking section of the train route on the basis of analyzing the defects of traditional relay circuit logic. This paper discusses the defects in the current design theory of traditional relay circuit and describes the implementation procedure of the improved method especially with an elaboration on its processing logic. This method can be used for such scenarios that the approach locking section should be extended up to or beyond the second route ahead of the train and can avoid the risk resulting from manually programming approach locking logic. Meanwhile, this method can also solve the problem of the train route wrongly converting to the approach locking state when the indication of a switch in the train route is lost.