Cesium-lead halide perovskite nanocrystals (CsPbX₃ (X = Br, Cl, I) PNCs) have become ideal luminescent materials for wide color gamut display devices, white LED lighting and high-efficiency solar cells, due to adjustable energy band gap, high fluorescence quantum yield, narrow fluorescence emission peak, and ultra-high defect tolerance. The preparation of CsPbX₃ PNCs with controllable size and morphology is a prerequisite for obtaining efficient and stable photovoltaic/photovoltaic devices. In this report, the CsPbBr₃ PNCs with different shapes are prepared by adding different concentrations of dodecanedioic acid (DDDA) ligands at room temperature through using ligand-assisted reprecipitation method. Utilizing the X-ray diffractometer, transmission electron microscopy, ultraviolet spectrophotometer, fluorescence spectrometers (PL), the phase structure, microstructure and optical properties of the nanocrystals are investigated. The results show that the presence of DDDA ligands have no influence on the phase structure of nanocrystal products, they all present a cubic phase structure. Surprisingly, the morphology of the nanocrystals gradually transforms from nanocubes into nanoplatelets with ～5 layers in thickness as the concentration of DDDA increases. In addition, the PL spectrum shows a significant blue shift from 509 nm to 478 nm. By using the in-situ homemade PL device with ultra-high time resolution (～100 ms), the real-time monitoring PL spectra of nanocrystals in the formation process are measured. The results demonstrate that nanocrystals undergo rapid nucleation and focusing of size distribution growth to generate nanocubes in the absence of DDDA ligand. When the DDDA ligand is present, nanocrystals are mainly nanoplatelets in the early growth stage due to the decelerated reaction. As the reaction proceeds, nanocubes can emerge and grow gradually while the nanoplatelets disappear when the concentrations of DDDA ligands are 25% and 50%. As the concentration is further increased to 75%, almost nanoplatelets could be formed after the nucleation stage and growth stage. Unexpectedly, preformed nanoplatelets are unstable for the prolonged reaction time as a result of the high surface energy, and they will eventually transform into isotropic nanocubes through dissolution-recrystallization pathway, indicating that the process in the later stage is controlled mainly by thermodynamics. Our findings offer an efficient strategy to synthesize the perovskite nanocrystals with controllable size and morphology.