This paper develops a hybrid quantum channel model tailored for finite-regime quantum key distribution (QKD) in free-space satellite quantum communication. The channel model addresses the fading characteristics of satellite-to-ground links while incorporating the dynamics of satellite orbits. By conducting a comprehensive security analysis under finite-size effects and collective attacks, the research bridges the theoretical and practical aspects of satellite-based QKD. Critical parameters, such as reconciliation efficiency, transmission coefficient, detection efficiency, receiver telescope aperture, beam waist, and effective communication time, are systematically investigated to optimize the secret key rate. The study reveals key trade-offs between the influence of satellite altitude on communication windows, noise levels, and synchronization, which significantly impact quantum channel efficiency. The model improves the accuracy of free-space quantum channel evaluations by presenting a hybrid noise framework that combines quantum Poissonian noise with classical additive white Gaussian noise (AWGN). Our numerical results demonstrate the feasibility and robustness of finite-regime QKD for uplink and downlink configurations in various atmospheric conditions, highlighting the superiority of satellite-based quantum communication over conventional networks.