Influences of large impact loads including high-speed railways, high-capacity arc furnaces and high-voltage high-capacity motors start-up on power quality of grid are researched deeply in this paper. Firstly, the characteristic and modeling of this three typical impact loads are studied. Secondly, simulation influences of these three loads on power quality of grid are implemented and compared with data measured from in-site. Finally, some corresponding simulation waves, measurement data and conclusions are given. These conclusions provide a basis for effective supervision of these loads and implementation of corresponding treatment measures, and have specific reference value.