The invention provides a probability trend calculating method considering a large-scale photovoltaic grid-connected power generation system. The method comprises the following steps of: 1) establishing a typical U-I characteristic mathematical model of a grid-connected large-scale centralized photovoltaic grid-connected power generation system; 2) based on an MPPT control algorithm, calculating a maximum output power value of the large-scale centralized photovoltaic power generation system under a typical day illumination intensity, and calculating a mean value and a mean square error of output power; 3) determining a system net frame, considering a power output constraint, and carrying out Monte-Carlo sampling according to probability parameters of a power supply and loads in a system; and 4) considering a node voltage constraint, and calculating a probability trend of the electric power system according to a Newton-Raphson algorithm. According to the real-time illumination intensity of a typical day of someplace, the photovoltaic power generation model can precisely calculate the output of the photovoltaic power generation system and can automatically modify a photovoltaic output equation according to real-time illumination data and environment temperature changes, the method is simple, and the practicality is high.