The yearly bided volume's occupancy factor of the whole volume in the northeast regional electricity market of China is about 80%， and that of the monthly bided volume is about 20%. When the yearly bid accomplished， we need to find a method to know the decomposition of the yearly bided volume to each month， then to get the monthly generation scheme. Generally， yearly bided volume of electricity in regional electricity market is averagely decomposed to each month of the year at hand. The average decomposition way has such disadvantages as going against the stability of monthly bidding market and the decrease of purchasing cost of the grid company. For this problem， this paper proposed a decomposition method of year bided volume based on the roll-uniformization of monthly competitive bidding space， and educes the relationship of purchasing costs between the uniformization and the non-uniformization of monthly competitive bidding spaces. Moreover， the impact of price drift on uniformization benefit is analyzed. Computation examples show that the decomposition method proposed can not only reduce the fluctuation of the price of monthly bided volume but also decrease purchasing cost of the grid company in monthly bidding market.