Abstract: Effective frontier mathematical analysis formula for multi-area multi-period power purchasing in electricity markets is built based on modern portfolio theory (MPT). The optimized model of power purchasing under conditional constraints is proposed by considering the influence of the characteristics and application constraints of multi-area multi-period power purchasing on the power purchasing combination decisions. In addition, as to such case that constraints make power purchasing not attain theoretical effective frontier, the key constraints-improved model is built by combing effective frontier mathematical analysis formula and the decision model with constraints to seek key constraints, by which power can be purchased with least cost through improved key constraints. The validity and applicability of the proposed model are verified through test cases, which show that proposed model has some reference value and guidance significance for obtaining power purchasing strategy for power grid corporation.