By taking uncertainties of wind power and supply-demand interaction into account

a wind power contract with a limited floating range of power exchange based on seller-side flexible electricity contract is proposed. The utility functions of both purchasing and selling sides in the spot market and contract trading are derived

and acceptable price ranges of contract based on the opportunity costs of contract trading are generated. The contract is finally formulated by over-the-counter negotiation and system security check. The wind power producer can participate in the bidding market while signing contract with customers

or mitigate the utility reduction from wind power uncertainties through multi-part alliance formed in the power generation or consumption rights trading. The numerical example is solved by the Monte Carlo simulation. The effectiveness of the method is verified

and the factors affecting the contract price are analyzed. Results show that the contract trading of wind energy can improve the profitability of all trading participators. This work is supported by State Grid Corporation of China (No. DZN17201300045).