The metro network is a significant part of the public transportation network, and measuring its robustness can help evaluate its performance when facing different operation incidents. Based on the shortest path between nodes, commonly used robustness parameters such as network efficiency neglect the variety of routes in the metro network. The perspective of node failure also ignores that metro trains can only turn back at stations with sidings, and a single node failure may lead to nearby normal nodes being out of service. In this paper, we define all possible routes chosen by metro passengers as effective routes, give two novel metro network robustness parameters, and a new perspective called section failure based on the sidings at specific metro stations. Taking the Chengdu metro as an example, we discuss its robustness when facing malicious continuous attacks. The result shows that the network efficiency parameter and the perspective of node failure underestimate the loss brought by metro operation incidents, and the Chengdu metro network is vulnerable when facing malicious attacks, especially based on node Laplacian gravity centrally. The paper gives a new method to analyze the robustness of metro networks and advises operators on protecting key metro stations to avoid large-scale failure.