In view of the influence exerted by nonlinear loads on the power quality of distribution networks, a preliminary analysis has been made of the operational characteristics of two typical kinds of nonlinear load models, i.e. single-phase uncontrolled rectifier filter circuit and single phase controlled rectifier filter circuit model, followed by the establishment of simulation models of these two typical nonlinear load models in MATLAB/SIMULINK. Based on these characteristics, the internal and external factors are modified to observe the power quality of the power supply and the load side. The simulation results show that the amplitude and phase of the harmonic currents are related to the amplitude and phase of each harmonic voltage as well as the conduction angle and the cut-off angle of the circuit. The results also show that the major factor responsible for the low power quality in distribution network lies in the fact that there are a large number of nonlinear loads, namely harmonic source, in distribution networks.