Abstract: High quality and large size bulk single crystals are essential to study the basic properties of perovskite materials and their performance limits of X-ray detection and imaging. In this paper, high quality MAPbI₃ and MAPbCl₃ single crystals were prepared using the silicone-oil assisted method and the properties and the performance of the single crystals and the corresponding X-ray detectors were characterized. These performances of single crystals grown with the silicone-oil assisted method are better than those prepared with the inverse temperature crystallization method(ITC). The ITC, as one of the most prevailing crystal growth methods, has drawbacks over the control of the nucleation and the growth rate. Silicone-oil assisted method was recently developed to overcome the above drawbacks and grew the perovskite single crystals in a controllable manner. In the silicone-oil assisted method, a capping layer on the precursor is adopted to reduce the solvent evaporation rate to a certain level and thus to manage the precursor in a slightly supersaturated condition. Silicone oil is selected as the capping layer for its smaller density and the insolubility with the precursor. The solvent in precursor will diffuse through the silicone oil capping layer and evaporate into the air at the silicone oil/air interface, driven by the content difference and the saturation vapor pressure, respectively. The silicone-oil assisted method realizes the constant growth of perovskite single crystals, since the driving force of the crystallization relies on the solvent diffusion rate but not the temperature-depended solubility. During the perovskite crystal growth process, the precursor is at a steady condition(constant temperature and constant concentration) in majority of the time. According to the classic growth theory, single crystals grown at a constant rate will have better crystal quality. Indeed, MAPbI₃ single crystals grown with the silicone-oil assisted method have a FWHM of rocking curves of (1 1 0) crystal planes of 0.011 2°, the fluorescence lifetime of 1 022 ns, and the lowest defect density of 5.61×10⁹ cm^-3. MAPbCl₃ single crystals grown with the silicone-oil assisted method have a FWHM of rocking curves of (1 0 0) crystal planes of 0.014 3°, the fluorescence lifetime of 957 ns, and the lowest defect density of 3.57×10⁹ cm^-3. The X-ray detector based on MAPbI₃ single crystals has a high sensitivity of 7 300 μC/(Gy·cm²). All results indicate that this silicone-oil assisted growth method is suitable for the mass production of high-quality single crystals for radiation detection.