Lead-free bismuth-based halide perovskites and their analogues have attracted research interest for their high stability and optoelectronic properties. However, the morphology-controlled synthesis of bismuth-based perovskite nanocrystals has been rarely demonstrated. Herein, we report the colloidal synthesis of zero-dimensional (0D) Cs₃BiCl₆ nanosheets (NSs), Cs₃Bi₂Cl₉ NSs/nanoplates (NPs) and Cs₄MnBi₂Cl₁₂ NPs through a hot-injection method. We demonstrate that the Cs₃BiCl₆ NSs, as an initial product of Cs₃Bi₂Cl₉ and Cs₄MnBi₂Cl₁₂ NPs, can transform into Cs₃Bi₂Cl₉ NSs or Cs₄MnBi₂Cl₁₂ NPs via Cl-induced metal ion insertion reactions under the templating effect of Cs₃BiCl₆. This growth mechanism is also applicable for the synthesis of Cs₄CdBi₂Cl₁₂ nanoplates. Furthermore, the alloying of Cd²⁺ into Cs₄MnBi₂Cl₁₂ lattice could weaken the strong coupling effect between Mn and Mn, which leads to a prolonged photoluminescence lifetime and an enhanced photoluminescence quantum yield (PLQY). As a proof of concept, the alloyed Cs₄Mn_xCd_1–xBi₂Cl₁₂ NPs are used as a scintillator, which show a lowest detection limit of 134.5 nGy/s. The X-ray imaging results display a high spatial resolution of over 20 line pairs per millimeter (lp/mm). These results provide new insights in the synthesis of anisotropic bismuth-based perovskite nanocrystals and their applications in radiation detection.