A linear supply function equilibrium（LSFE） model considering network constraints and demand side bidding for oligopolistic electricity market is presented. In this market model， the ISO solves an optimal power flow for dispatching generation and determining nodal prices， and participants will choose their bids to seek the maximum profits. Developed from agent-based simulation methods， the coevolutionary approach simulates the coevolutionary mechanism in nature and adopts the notions of ecosystem. It is employed to calculate the Nash equilibrium point in this work. Numerical examples are used to validate the effectiveness of the proposed method. Simulation results show that the robust and flexible of the coevolutionary approach. It can converge to pure strategy equilibrium rapidly if it exists.