In this study, ZnO crystals have been synthesized via hydrothermal method by adjusting the component of the precursor with the pulsed magnetic field. The effects of the molar ratio of the reactants and pulsed magnetic field applied in the synthesis process on the microstructure and optical properties of ZnO crystals were investigated systematically. The samples were characterized by X-ray diffraction, scanning electron microscopy, UV-Vis spectrophotometer and micro-Raman scattering. The results demonstrate that pulsed magnetic field increases the defects concentration and reduces the band gap of ZnO. And it also promotes nucleation and crystal growth, the crystals obtained under pulsed magnetic field are more uniform and complete. The change of molar ratios influences the pH value so that the environment for the growth of ZnO crystals changed dramatically.