The pattern of the power industry will be restructured in the market reforms: the power generation plan and the sale market would be opened to every market participants. In this mode, the power generation and consumption behavior will be more difficult to capture and predict than in the environment of monopolistic buying and selling model. Power grid planning is going to be challenged. In order to adapt to the new situation, it is necessary to actively adjust the power grid planning thoughts, and explore some innovative load forecasting methods. The improvement method of load forecasting is put forward on two aspects. Firstly, the price response factor is added into the long term load forecasting method. Secondly, the market transactions are simulated based on the forecasting results of load distribution, power supply planning and marginal price, thus the power balance and transaction price level of the long term power market can be captured, and then the trading results can conversely corrects the load forecasting results. Shown as the case study, the method takes full account of the market participants’ elastic response to the transaction price, and brings a certain reference value to the power grid development planning in the power market environment.