Abstract: Large scale grid-connection of such intermittent power supply as wind power and photovoltaic (abbr. PV) power leads to higher requirement on power system flexibility. To construct a power system containing high proportion of renewable energy sources and mine the regulation ability of flexible generation units such as energy storage, hydropower and thermal power as well as that of flexible resource such as load response and so on, an optimal allocation method of power system flexible resources, in which the demand side response was considered, was proposed to make the planning scheme enable to implement the economic optimization while the flexible demand was satisfied. In the proposed allocation method the planning layer, operation layer and flexible layer were overall considered, and taking the minimized cost as objective the power source planning model, short-term operation simulation model and ultra-short-term operation simulation model were solved by multiple iterations. Besides, combining with two kinds of demand response means, i.e., peak-valley-flat TOU price and interruptible load, both economy and flexibility of system operation were further enhanced. In the solving method the robustness of the calculation was improved by combining multi-level iteration with relaxation technique. Taking a certain regional power network for example, the effectiveness of the proposed allocation method is verified.