The present invention provides a smooth output method for a combined power generation system of a wind farm group and a thermal power plant, comprising the steps of: acquiring a data set consisting of a wind power generation power predicted value of each wind farm in the wind farm group, and adding all the wind power generation power predicted values in the data sets corresponding to the all wind farms to obtain a comprehensive data set consisting of total power generation power of the wind farm group; fitting the comprehensive data set through a polynomial fitting algorithm to obtain a wind power smooth output formula, and calculating a wind power smooth output value; calculating the sum of the wind power smooth output value and a standard output value of the thermal power plant, so as to obtain a total smooth output value; and determining the actual output conditions of the wind farm group and the thermal power plant according to the total smooth output value, the total power generation power of the wind farm group, and the minimum output value, the maximum output value and the standard output value of the thermal power plant. According to the smooth output method of the present invention, the obtained smooth output curve does not delay, and low energy consumption, little pollution, and stable electric energy output can be ensured.