The invention discloses a high pressure and low pressure bypass system. The high pressure and low pressure bypass system comprises a high pressure bypass system connected with a high pressure cylinder, a low pressure bypass system connected with an intermediate pressure cylinder and a low pressure cylinder, a circulation pipeline and a condenser. The high pressure bypass system comprises an overheating pipeline, a backheating pipeline and a high pressure bypass pipeline. The low pressure bypass system comprises a reheating pipeline and a reheating condensation pipeline. The circulation pipeline comprises a circulating water pipeline and a high pressure temperature reducing pipeline. The high pressure and low pressure bypass system has the advantages that the structure is reasonable, the service life is long, unit load adaptability is improved, and a unit can be suitable for frequent start and stop and rapid load rise and fall; under the working condition that a turbine is started or load shedding is conducted, the temperature and the pressure of live steam are reduced through the bypass system, and then the live steam is fed into a reheater, dry burning of the reheater is avoided, and therefore the reheater is protected; in the sudden shedding of part of or all of loads of the unit, a bypass is quickly opened, a working medium is recycled to the condenser, running stability of a boiler is changed, and motion of a safety valve is reduced and even avoided.