ABSTRACT: Large-Scale volatility photovoltaic grid-connected which will have a profound impact on the reliability of the system. A new method based on maximum likelihood estimation is presented in this paper in which probabilistic distribution of solar irradiance is calculated on the premise of solar irradiance beta distribution, according to volatility and intermittent characteristics of photovoltaic, and simulated the actual solar irradiance which affecting power output of photovoltaic based on Monte Carlo method, then, Established a photovoltaic probability output model which can reflect the randomness output of photovoltaic; and use of the partitioning method, the reliability assessment of an interconnected generation system with photovoltaic connected with IEEE-RTS79 system is performed by Monte Carlo simulation; The results show that there have a contribution on reliability evaluation of the grid when connected on determined location and under a certain load level; also the example show the influence of different parameters of solar irradiation on system reliability indicators and the result show different parameters of beta distribution have a great impact on grid reliability indicators.