The power-enhancement technology of GNSS is one of the effective measures to improve the anti-jamming capability of regional satellite navigation signals. How to select power enhancement sub-constellation which has less satellite quantity and better service performance from global constellation is a problem in urgent need to be solved urgently in the construction of modern navigation system. An optimal design method of power-enhanced sub-constellation based on minimum number of satellites was proposed. The design process, mathematical model and optimal solution search strategy were introduced. Availability level, accuracy level and coverage area were defined to evaluate the performance of the power-enhanced sub-constellation. Taking GPS as an example, the optimal design and performance evaluation of power-enhanced sub-constellation was carried out for the application background of covering point target and regional target. The results of simulation show that power enhancement at any target point worldwide requires 12~17 satellites. 18 satellites are needed to achieve continuous coverage to the coastal area of China. And, covering the entire Asia-Pacific region requires full constellation satellite enhancements to meet continuity and accuracy requirements. In this case, the service range of optimal power enhanced sub-constellation can be extended to the whole world.