According to the wide-speed-range variable-cycle engine for driving the low-pressure turbine rotor based on the interstage combustion chamber, firstly, the interstage combustion chamber is opened, a turbofan interstage afterburning cycle mode is started, the thrust of the engine is greatly increased, meanwhile, all the first mode selection valves are opened, and the second mode selection valve is closed; therefore, more air flow with high oxygen content enters the internal duct afterburner, an internal duct afterburner mode is started, and Ma0-2.5 + flight is realized. When Ma reaches 2.5-6, a turbine engine core engine is closed, meanwhile, first modal selection valves are all opened, a duct second modal selection valve at an inlet of an interstage combustion chamber is gradually opened, a part of air pressurized by a fan enters the interstage combustion chamber I through the second modal selection valve, a low-pressure turbine is driven to drive a fan A to rotate and pressurize, and the fan A is driven to rotate and pressurize; after air flow pressurized by the engine fan A enters the outer duct, the dual-mode ramjet engine is started and enters a stamping mode, and Ma2.5-6 flight is achieved.