In order to explore the reactive voltage characteristics of power distribution networks affected by the grid-connected photovoltaic (PV) system，a probabilistic distribution model of PV power generation system is estab-lished based on the Beta-distribution of solar irradiance． Then，by taking into consideration the probabilistic impact of solar radiance on the output of PV power generation system，the PV outputs in three typical weather conditions，namely a low cloudiness day，a cloudless day and a partly cloudy day，are simulated，and the probabilistic output of PV power generation system in continuous time sections is obtained，which is used to perform the power flow cal-culation in turn． Moreover，the calculated power flow is compared with that obtained by using the constant power model． Finally，an example is given to analyze the laws of voltage distribution and voltage amplitude fluctuation in grid-connected PV distribution networks． The results also show that，as compared with the conventional constant power model，the established probabilistic model reflects the randomness and volatility of PV output better．