The invention discloses a method and a system utilizing external steam for start debugging of a new-built supercritical unit. The method comprises steps as follows: a first isolation gate is additionally mounted between a high-pressure bypass valve I and a reheat cooling section I of a production unit, a communication pipeline is additionally mounted at an isolation section between the high-pressure bypass valve I and the reheat cooling section I of the production unit, a communication gate is arranged on the communication pipeline, and a second isolation gate is additionally mounted between a high-pressure bypass valve II and a reheat cooling section II of the new-built supercritical unit; the first isolation gate is closed, the communication gate is opened, and external steam after temperature and pressure reduction is supplied to the new-built supercritical unit by the high-pressure bypass valve I of production unit; a superheater and a connecting pipeline are purged under the reduced pressure by means of the external steam, and a reheater and a connecting pipeline are purged under stable pressure and reduced pressure by means of the external steam; overall start debugging of a turbine of the new-built supercritical unit is performed. Under the condition that a boiler of the new-built unit is not ignited, steam purging of the units and overall start debugging work of the no-load test operation stage are completed by the aid of the external steam provided by the production unit, so that the ignition times of the boiler in the supercritical unit and time of operation in a non-economic area are reduced.