Before the spot electricity market is established, electrical energy is traded through forward contracts in the current medium and longterm electricity market in China. A reasonable balancing and settlement mechanism for electrical energy deviation is conducive to the smooth transition from the mediumand longterm electricity market to the spot market. Given this background, an assessment mechanism is presented for contract energy deviation in a mediumand longterm electricity market. First, the deviation settlement mechanism of the basic rules for the mediumand longterm electricity market is examined, and a generationside deviation balancing mechanism suitable for the current electricity market environment proposed. Then, considering the differences of the marketoriented electricity proportion and the differences in the capabilities of various users in controlling contract deviations, the risk of the electrical energy deviation penalty imposed on demand customers is analyzed. A fixedratio energy deviation settlement method is next designed for nontotal electrical energy transaction, and a staged assessment mechanism of demandside energy deviation proposed. Simulations based on actual deviation assessment data from a provincial electricity market in China show that the proposed energy deviation assessment mechanism is effective in the mediumand longterm electricity market.