All-inorganic CsPbBr3/Cs4PbBr6 perovskite composite nanocrystals (NCs) have been successfully preparaed by one-step method. CsPbBr3/Cs4PbBr6 NCs were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), ultraviolet-visible spectroscopy (UV-Vis), and photoluminescene spectroscopy (PL). The effects of reaction temperature, reaction time and Cs/Pb ratios on the phase, morphology and optical properties of NCs were systematically studied. The results show that the samples are cube-like morphology with great dispersion and high crystallinity, and the size of the NCs is about 20 nm. The PL intensity of NCs could be significantly increase then decrease with the increase of the reaction temperature and reaction time, and the optimal reaction temperature and time is 90 ℃ and 30 min, respectively. Futhermore, the molar ratio of Cs/Pb exhibited a great influence on the phase composition and emission property of NCs. The relative content of Cs4PbBr6 phase gradually increases with the increase of Cs/Pb ratio, which could served as a protective layer for CsPbBr3, resulting in the increase of emission intensity. The flourescence intensity reaches the max value when Cs/Pb=2∶3. However, excessive molar ratio of Cs/Pb induce the decrease of emission intensity due to the suppressed formation of CsPbBr3.