In order to overcome the neglect of medium- and long-term costs while underestimating short-term investment in current power systems in economic evaluation， a 3-dimensional life cycle cost（LCC） model for the power system as a whole is developed in the perspective of the component dimension， the cost dimension and the time dimension. The cost dimension is studied by structurally analyzing the device layer， the system layer and the cost of surroundings. A series of strategies for economic evaluation based on LCC is presented. The conversion methods are studied for devices with different life cycles. Multistage calculation is applied for devices with different runtimes. The traditional economic evaluation indicators are improved and the efficiency indicators based on LCC are proposed. Case studies are made on an actual 500 kV substation and 110 kV distribution network， respectively. It is shown that the economic evaluation results are much more accurate and valid， providing a reference for further deepening asset management of power systems.