The invention discloses a system for preventing backflow of circulating water into a condenser in a thermal power plant, belongs to the technical field of boiler drainage systems in thermal power plants. By arranging a check valve and a detachable pipe section, the possibility of backflow of the circulating water into the condenser is fundamentally eradicated, the circulating water can be prevented from flowing back into the condenser with low initial cost, and therefore the occurrence of huge economic loss accidents to a unit is avoided.