The security of practical quantum key distribution (QKD) systems based on weak coherent pulse (WCP) sources is imperiled by photon number splitting (PNS) attacks due to imperfectness of devices and channel loss.The relation between the zenith angle and the mean photon number for security of a satellite-to-ground QKD system based on the WCP source against PNS attacks which operates with the optimal eavesdropping strategy is provided.The theoretical and calculation results shows the upper limits of key parameters for satellite-to-ground QKD systems,such as the zenith angle and the mean photon number,are limited by PNS attacks,and eventually the key exchange rate and the capacity of QKD systems are limited as well.The zenith angle and the mean photon number have an incompatible effect on the capacity of a satellite-to-ground QKD system.At the same time a method of parameter estimation of the zenith angle and the mean photon number for security of practical satellite-to-ground QKD systems against PNS attacks is provided.