Abstract: One of the measures of Space Debris Mitigation is to execute a deorbit maneuver after the mission of LEO completed, and the satellites die out gradually by the drag of atmosphere in 25 years. Some fuels are needed in deorbit maneuver, so deorbit maneuver must be considered in the phase of satellite overall design. The relations of orbital parameters of LEO, cost of deorbit maneuver and existent lifetime must be studied deeply. The theories and methods of solving deorbit maneuver are explained. A highly effective method of calculating orbit lifetime is introduced： method of differential-integral, and its correctness and validity are validated. The relation of aftermission orbit lifetime and expense of deorbit maneuver is studied. The positive question and the contrary question are presented and solved. The infections of other factors such as F10.7 and s acting on those relations are studied, and estimate values are provided. Some computing results are provided, and are analyzed and summarized. An applied method dealing with question of deorbit maneuver is provided for the design of LEO.