The invention relates to a heating performance calculation method for a fuel gas-steam combined cycle unit. The method comprises the steps that 1, through a heating performance test in a back pressure mode, a function relation between heating steam quantity, heating quantity and gas turbine power under the condition that the unit runs once by dragging twice in the back pressure mode and a function relation between the heating steam quantity, the heating quantity and the gas turbine power under the condition that the unit runs once by dragging once in the back pressure mode are obtained; 2, according to heating performance in an extraction condensation mode, a mathematical model of the group load and heating capability of unit in the extraction condensation mode is established; 3, according to the model, the heating performance and peak regulation performance of the combined cycle unit are obtained. Taking an F-class fuel gas-steam combined cycle unit in an area as an example, by means of conducting tests and mathematical modeling, the heating performance and peak regulation performance of the typical combined cycle unit in various running modes are obtained, and data support is provided for power grids and heat supply networks to better conduct coordinate control.