1. A multi-domain routing computation method, comprising: when it is determined that there are two or more domains deployed with a Path Computation Elements (PCE) in a network, computing a routing path, between a PCE of a first domain and a PCE of a last domain among the two or more domains deployed with a PCE, using an extended Backward-Recursive PCE-Based Computation (BRPC) algorithm, and computing a routing path using a Routing Controller (RC) in domains other than the two or more domains deployed with a PCE; wherein the computing a routing path, between a PCE of a first domain and a PCE of a last domain among the two or more domains deployed with a PCE, using an extended BRPC algorithm comprises: after a PCE receives a trigger request for path computation, determining whether a domain where the PCE is located is the last domain, if it is the last domain, computing a Virtual Shortest Path Tree (VSPT) of said domain: if it is a non-last domain, further determining whether there is a domain deployed without a PCE between said domain where the PCE is located and a next domain deployed with a PCE downstream of a sequence of domains, if there is not, computing the VPST of said domain; if there is, taking, as extended link topology, an intra-domain abstract link of said domain deployed without a PCE and inter-domain link(s) between domains deployed without a PCE, combining the extended link topology with a link topology of said domain where the PCE is located, and computing the VSPT after the combination of link topology.