The invention discloses a control method and system of a steam exhaust and heat supply system of a back-pressure steam-driven induced draft fan small steam turbine. The control method comprises the following steps: judging whether a second regulating valve is mounted on a four-stage steam extraction pipeline, connected with a deaerator, of a machine set or not and whether the second regulating valve is in the normal state or not; if the second regulating valve is mounted on the four-stage steam extraction pipeline, connected with the deaerator, of the machine set and is in the normal state, switching the heat supply control mode to the normal heat supply condition, so that during the steam exhaust process of a back-pressure steam-driven induced draft fan small steam turbine unit, the priority is given to the external heat supply through a heat supply steam distribution cylinder, and the steam supply to the heating of the deaerator is complementary; and otherwise, switching the heat supply control mode to the temporary heat supply condition, so that during the steam exhaust process of a back-pressure steam-driven induced draft fan small steam turbine unit, the priority is given to steam supply to the heating of the deaerator, and the external heat supply through the heat supply steam distribution cylinder is complementary. The control system comprises computer equipment used forexecuting the above steps of the control method through programming. By the adoption of the control method and system, the atmospheric steam exhaust problem, which occurs when the machine set is provided with high loads, of the back-pressure steam-driven induced draft fan small steam turbine can be effectively solved; the heat supply flexibility and the economic benefit of a thermal power generation enterprise which adopts a back-pressure steam-driven induced draft fan can be improved; and the purpose of energy conservation and emission reduction can be fulfilled.