FAPbI3 has garnered significant attention as solar cell material due to its exceptional properties. However, its crystal structure disorder at room temperature restricts the further improvement of device performance. Our previous research reported efficiency and stability are improved by MAPbBr3 incorporation. However, how MAPbBr3 improves the device performance in terms of charge carrier dynamics has not yet been fully understood. In this regard, we comprehensively elucidated improved optoelectronic properties in mixed perovskite thin films and its correlation with device performance via various spectroscopic techniques. According to crystallinity analysis, the disorder of mixed perovskite film exhibits check mark shaped composition dependence with a minimum at 0.8 mol%. In addition, from time-resolved measurement including transient absorption spectroscopy and optical-pump THz prove spectroscopy, it is demonstrated that charge carrier dynamics and optoelectronic properties have similar dependence. Particularly, 0.8 mol% shows less carrier loss, prolonged recombination dynamics and enhanced carrier transport properties. Lastly, the device performance captures the composition dependence and champion PCE is achieved at 0.8 mol%. This comprehensive investigation shows the importance of understanding the charge carrier dynamics and provides novel insight for optimizing the composition of mixed perovskite to further improve the PCE and stability of the perovskite based solar cell.