The invention relates to a method for calling route in tandem system and relative tandem system. Wherein, it is characterized in that: between the first and second tandem exchangers based on soft exchange, setting the interface of descending calling communication; the first tandem exchange transmits received descending calling to connected second tandem exchange via said interface, and the second tandem exchange processes calling route on said descending calling. The invention can supply at least two routes to the descending calling, to realize route backup and load share between several descending calling routes. Said interface can use SIP-T protocol, to realize the route backup and load share between different or same tandem exchanges, without adding additional physical device and connections, to reduce the realization cost and maintenance cost, and improve the reliability of descending calling.