In the electricity market with user side opening, utilizing aggregated electric loads as system resources has several benefits. Electric water heaters can participate in power dispatch, such as peak-load and frequency adjustment. By the centralized control of large electric water heater, users can offer bidding strategies to get more profits. Assuming that the power of water heater involved in dispatch can be smoothly adjusted. This paper analyzes the cost that user participating power dispatch and the relationship between profit, market clearing price and bidding strategy coefficients. Finally gets a single electric water heater user's optimal bidding strategy. Examples and conclusions in the text verified the validity of the bidding strategy.