In the unilateral electricity market competition among power suppliers， bidding of the large-scale electric quantity in the medium and long-term contract market may cause great market risk. Based on the analysis of the influence of the proportion of competitive electric quantity in medium and long-term contract electricity market and the proportion of year contract（bilateral transaction） to price formation mechanism， a model for calculating the proportion of competitive electric quantity in medium and long-term contract electricity market is put forward to ensure the competitive formation of market price， which uses the index named must run rate of generation companies to evaluate the market power in electricity market with fully considered the factors of competitor status， market supply and demand situation as well as the units' availability. With a simulation example， it is proved that the proportion of competitive electric quantity calculated by the proposed model can guarantee that there have no generation company has the ability to decide the price of any part of its offered electric quantity in medium and long-term contract transactions， thus guarantees the effective market competition.