Power grid system is one of the most important and complex infrastructure systems in our society. Catastrophic blackouts caused by cascading failures, however, still occur and lead to devastating effects. Here, for the first time, we investigate the robustness of the Chinese power grid (CPG) system under various attack and defence scenarios by considering cascading failures. The simulation results illustrate that the effect of the loading level of the power grid network has a large influence on the cascading process. When the initial load of the edge is smaller (α≤1.6), the ascending order strategy of the edge collective influence is the most effective attack strategy for triggering cascading failures over the CPG network. While when the initial load of the edge is larger (α≥1.6), the high load strategy appears to outperform the other strategies. To protect the national power grid from cascading failure, two approaches can be adopted: one approach is to improve the network load capacity, and the other approach is to protect the critical edges. The efforts needed to improve the cascading robustness of the CPG network within these two approaches are quantified. Our results can guide protective measures to avoid large cascading failures and are useful for the design of a robust national power grid network.