The invention discloses a low-pressure cylinder combined zero-power heating supply system and method in a thermal power generating unit, and relates to the field of thermal power plant heating supplyenergy conservation. The low-pressure cylinder combined zero-power heating supply system comprises an intermediate-pressure cylinder, an intermediate-pressure control valve, low-pressure cylinders, aheating network heater, a condenser, a control regulating valve and a cooling steam system. The intermediate-pressure control valve is arranged on an intermediate-pressure cylinder steam inlet pipeline. A steam inlet of the heating network heater is connected with an intermediate-low pressure cylinder communicating pipe and the intermediate-pressure cylinder steam inlet pipeline. The pipeline is provided with regulation valves. A heating supply steam extraction pipeline led from the intermediate-pressure cylinder steam inlet pipeline is provided with a desuperheating and reducing device. The cooling steam system is connected with a low-pressure cylinder steam inlet pipeline. A cooling steam pipeline is provided with a regulating valve, a flowmeter and temperature and pressure measuring points. A low-pressure cylinder steam outlet is connected with the condenser. The intermediate-low pressure cylinder communicating pipe is provided with a regulating valve with the sealing function. According to the low-pressure cylinder combined zero-power heating supply system, the peak load regulation capacity of the unit can be further improved under the condition of meeting heating supply requirements, and the system further has the characteristics of being small in investment and flexible to operate.