The invention relates to the technical field of steam turbines, in particular to a single-reheating steam turbine of parallel-connection high pressure cylinders. The single-reheating steam turbine of the parallel-connection high pressure cylinders comprises a main steam turbine system and an auxiliary high pressure cylinder; the main steam turbine system comprises a high pressure cylinder, an intermediate pressure cylinder, low pressure cylinders and a reheater; a steam inlet of the high pressure cylinder is communicated with a high pressure steam pipeline, and an exhaust port of the high pressure cylinder is communicated with an inlet of the reheater through a cold reheating steam pipeline; a steam inlet of the auxiliary high pressure cylinder is communicated with the high pressure steam pipeline through an auxiliary steam inlet pipeline, and an exhaust port of auxiliary high pressure cylinder is communicated with the cold reheating steam pipeline through an auxiliary exhausting pipeline; the auxiliary steam inlet pipeline is provided with an auxiliary steam inlet valve; an outlet of the reheater is communicated with a steam inlet of the intermediate pressure cylinder through a hot reheating steam pipeline; the high pressure cylinder is provided with a high pressure steam extracting port; a first steam extracting pipeline leading to a first heater is connected to the position of the high pressure steam extracting port; and the first heater is connected with a first bypass in parallel. The economic performance of the work condition of a part of loads can be improved; and meanwhile, the economic performance of the work condition of high loads is improved, and the economic performance of the work condition of rated loads is guaranteed.