Voltage quality for residents of 10kV and below is affected by some factors: voltages of substation 10kV buses, power supply topologies, line types, and power supply distances of 10kV lines, reactive power compensation and loads of distribution transformers. As loads of distribution transformers vary randomly in space and time, voltages of distribution transformers fluctuate randomly as well as distribution network power flows, even network structures and parameters are invariable when voltages of 10kV buses. According to the definition of voltage eligibility rate, a random fluctuation model of distribution transformer loads was built through long term statistical analysis of distribution transformer loads on actual distribution network. By calculating the probabilistic load flows and considering correlation of random fluctuations, static voltages of distribution transformers were analyzed with probability method and power supply voltage eligibility rates of all 10kV lines could be calculated. The simulations show that the power supply voltage eligibility rates can be analyzed and evaluated more comprehensively by probabilistic load flow calculation, such a calculation provides the theoretical calculation basis for both reasonably controlling the bus voltages and improving the power supply voltage eligibility rates.