Ultrathin 2D perovskite nanoplatelets (NPLs) have many excellent optical properties including narrow absorption and emission spectra, and large exciton binding energies. Doping Mn2+ into perovskite NPLs also introduces strong orange luminescence due to the d-d de-excitation recombination. However, it is very challenging to synthesize Mn2+ doped ultrathin perovskite NPLs. Here, we report the successful development of a room temperature method for Mn ion doped perovskite NPLs. The impact of the Mn2+ concentration on their optical properties has been systematically investigated. The highest PLQY is up to 71% when the Mn2+ doping level is 38.6%. Furthermore, we have observed a self-purification effect of these n = 2 NPLs, where the Mn ions were ejected from the n = 2 NPLs and injected into the n = 3 NPLs. An efficient energy transfer from the n = 2 host to the n = 3 NPLs has also been found. Additionally, we have used this fast ejecting and injecting property to fabricate an anti-counterfeiting film. The film shows weak blue and strong orange color under room temperature and high temperature, respectively. Most importantly, the process can be repeated several cycles without damage, which shows great potential for anti-counterfeiting applications.