All-solid-state thin film lithium battery (TFLB) is regarded as the ideal power source for microelectronic devices. However, the relatively low ionic conductivity of amorphous solid-state electrolyte limits the improvement of electrochemical performance for TFLB. In this work, amorphous lithium silicon oxynitride (LiSiON) thin films are prepared by magnetron sputtering as solid-state electrolyte for TFLB. With optimized deposition condition, the LiSiON thin film exhibits a high ionic conductivity of 6.3×10^-6 S∙cm^-1 at room temperature and a wide voltage window over 5 V, making it a suitable thin film electrolyte for TFLB. A MoO₃/LiSiON/Li TFLB is constructed based on the LiSiON thin film electrolyte with large specific capacity (282 mAh∙g^-1 at 50 mA∙g^-1), good rate capability (50 mAh∙g^-1 at 800 mA∙g^-1), and acceptable cycle life (78.1% capacity retention after 200 cycles), demonstrating the feasibility of this electrolyte for practical applications.