Mn-doped perovskite CsPb(Cl/Br)(3) nanocrystals (NCs) have been widely explored due to their unique dual-color emission characteristics, which could reduce the usage of toxic Pb and introduce the exciton emission in blue region and the stable Mn2+ emission peak in orange-red region via energy transfer from exciton to Mn2+ emission. Although doping high concentration Mn2+ could increase the Mn2+ emission, excess Mn2+ ions would be expelled from the host perovskite lattice. Doping secondary metal Ni into the NCs as an efficient method is always used to improve the Mn2+ emission. We adopted a one-pot halogen injection into Cs-precursor under ambient environment method to prepare (Mn, Ni): CsPbCl3 and (Mn, Ni): CsPb(Cl/Br)(3) NCs, which introduces NiCl2 in the Mn: CsPb(Cl/Br)(3) NCs to supply enough halide ions and to passivate defect states or traps in the perovskite NCs so as to enhance Mn2+ emission. Density Functional Theory (DFT) calculations were also conducted to explain our experimental results, which revealed that Ni2+ doping could eliminate the in-gap deep defects of the CsPb(Cl/Br)(3) nanocrystals. In addition, the passivated lattice defects help to improve the stability of the perovskite NCs, preventing from electron beam irradiation.