The Kerr rotation of MnBixAl0.15(0.4≤x≤0.9) thin films with and Al protective layer as a function of the Bi concentration, x, has been investigated. Compared with MnBix thin films, it is found that a large enhancement of Kerr rotation appears in the MnBixAl0.15 thin films with 0.4≤x≤0.7, but no enhancement of Kerr rotation appears when x is greater than 0.7. When x=0.5, a maximum Kerr rotation of 2.75° is observed at 633nm for the MnBi0.5Al0.15 thin film, which is much larger than that of 1.56° for the MnBi0.5 thin film. For the MnBixAl0.15 thin films with 0.4≤x≤0.7, the c lattice shrinking may result in a stronger hybridization between Bi 6p and Mn 3d states, which also should be responsible for the large enhancement of Kerr rotatio. In addition, the saturation magnetization Ms is reduced from 4×105A/m for the MnBi0.5 thin film to 3×105A/m for the MnBi0.5Al0.15 thin film, suggesting that some of Al may also substitute for Mn at the octahedrtal sites.