Taking Mianhuatan reservoir as an example, staged optimization operation without reducing annual flood control standard is implemented. Traditional hydrological frequency method is used to calculate stage design flood, and probability combination should be consistent with annual flood probability. Two variable sequences between staged design flood control capacity and frequency are established. The flood risk is calculated according to flood control capacity under staged limited water level. Under the premise that risk combination satisfied annual flood control standard, long series simulation is conducted based on power generation and flood regulation rules, to seek great overall benefit by transferring stage's risk and benefit. The results show that, limited water level in main and secondary flood season is 168.78 m and 169.58m respectively, flood risk rate is namely 1.217% and 0.783%, whose combination risk ratio is 1.99%, consistent with annual flood control standard 2%，and has greate benefit.