Abstract: The thermal exposure tests at 675℃ and for 1 000，3 000 h and 5 000 h respectively are conducted to the GH2070P ferronickel-based hi-temperature alloy tube as key candidate material for the boiler with 675℃ parameter，and the post thermal exposure mechanical properties and microstructure evolutions of the alloy are investigated. The facts as follows are revealed with the alloy tube as thermal exposed at 675℃ and for 1 000 hours. The γ' phase is precipitated in grain and the M₂₃C₆ carbide phase is precipitated at grain boundary；the ambient temperature tensile strength，the yield strength and the hardness of the alloy are obviously increased，whereas it’s impact toughness goes down；along with the increase of thermal exposure time going up to 5 000 h，the size of the γ' phase is coarsened，which is in alignment with the LSW coarsening theory；and the size of the M₂₃C₆ carbides are increased slightly，but are still presenting an intermittent distribution state at the grain boundary，and no network structure is formed；In general the ambient temperature tensile strength，the yield strength and the hardness of the alloy consistently keep at a rather high level，while its impact toughness reduction speed is getting slow.