Abstract: With the development of hydropower engineering toward high-altitude and high-stress environments, there is an increasing demand for structural steels possessing gigapascal-level strength and excellent low-temperature toughness. Tempering, as key heat treatment process, plays a vital role in balancing strength and toughness in hydropower steels. Taking 1 000 MPa low alloy steel for hydropower as the research object, the microstructure evolution and corresponding mechanical properties of the steel tempered at 590-680 °C were systematically analyzed by means of scanning electron microscope （SEM）, transmission electron microscope （TEM）, tensile and impact tests. The results indicate that as the tempering temperature increases, the as-quenched lath martensite undergoes recovery and gradually widens, with its lath characteristics continuously weakening. The microstructure ultimately transforms into a stable structure dominated by equiaxed ferrite and spheroidized cementite. Concurrently, the grain orientation distribution tends to become more randomized, the proportion of high-angle grain boundaries increases, and the dislocation density continuously decreases. Correspondingly, the yield strength and tensile strength show a declining trend, while the elongation continuously improves. The impact absorbed energy exhibits a non-monotonic change, which increases first and then decreases, and rises again at 680 ℃. Quantitative analysis of strengthening mechanisms reveals that as the tempering temperature increases, the contributions of grain boundary strengthening, dislocation strengthening, and precipitation strengthening to yield strength all show a continuous decreasing trend. The tested steel achieves optimal comprehensive performance under the 620 ℃ tempering condition, with a yield strength of 947 MPa, tensile strength of 983 MPa, elongation of 16.3%, and impact absorbed energy of 158 J at -60 ℃. The research can provide theoretical support and microstructure control basis for the optimization of heat treatment process of high performance hydropower steel, and has certain guiding significance for the development and engineering application of high strength and toughness hydropower steel.