The invention relates to a trend calculating method used for an electric power system containing photovoltaic and thermal loads. The method comprises the following steps of collecting active output data in a distributed photovoltaic power supply system and thermal load power in a meteorological sensitive load system and taking as variables; calculating an edge cumulative probability distribution function of each variable; according to the collected variables and the edge cumulative probability distribution function, through calculating a Copula function, acquiring joint probability distribution; according to the joint probability distribution, randomly generating a specified quantity of edge cumulative probability distribution values of each variable, and combining the edge cumulative probability distribution function so as to acquire a simulation sample value of each variable; and taking the simulation sample values of all the variables as input and carrying out trend calculating. Compared to the prior art, by using the method of the invention, meteorology correlation between a meteorological sensitive load and photovoltaic output is fully considered, simulation accuracy of an electric power system and energy integration utilization efficiency are increased and there are other advantages.