Organic metal halide perovskite is one of the most promising candidates for next generation solar cell material. Recently, the photo-conversion efficiency of perovskite based solar cell is achieved 25.7%. However, the photo-physics in perovskite is still unrevealed. In this work, we studied composition dependent charge carrier dynamics in mixed perovskite fim, (FAPbI3)1-x(MAPbBr3)x with x=0, 0.5, 0.8, 1, 2.5, 10, 20 mol%, with optical pump-THz probe (OPTP) and transient absorption spectroscopy (TAS) measurement. We revealed that small amount of MAPbBr3 additive improves effective mobility and diffusion length by crystal phase stabilization effect. However, further increases of MAPbBr3 drastically reduce them by disordering effect. From TAS measurement, we monitor modulation of the band structure of mixed perovskite film. Band gap is monotonically broadened and additional transition states are emerged at high composition. Moreover, low MAPbBr3 content shows low carrier loss in the thermalization process and slow mono and bi-molecular recombination. Such enhancement of properties of the mixed perovskite film is maximized at 0.8 mol%. Therefore, our results well explain the highest photo-conversion efficiency reported recently with 0.8 mol% of MAPbBr3 additive. In addition, our observation is helpful for efficient compositional engineering of mixed perovskite to further improve the photo-conversion efficiency and stability.