The present invention discloses a method for implementing cross-domain constraint routing, through setting in addition the border node identifier in the ERO sub-objects saved in ERO corresponding to nodes that a route will pass, this invention realizes the distributed calculation of the route. When calculating the path of cross-domain constraint routing, the original node determines the first border node that the route will pass based on the border node identifiers set in the ERO sub-objects in ERO, calculates the path to the first border node, and then sends a PATH message; the first border node determines, based on the next ERO sub-object with a border node identifier in the ERO of the received PATH message, the next border node that the route will pass, calculates the path to the further next border node, and then sends the PATH message; . . . Repeat such a process until the PATH message reaches the destination node.