In present work, the hardenability and critical cooling rates of 7B50 alloy were investigated based on measured cooling curves obtained from the spray quenching test on modified Jominy specimen, electrical conductivity and hardness in different aging tempers, and microstructures evolution in natural aging temper. Results show that the hardened depth of 7B50 alloy is about 70 mm in natural aging temper while it decreases to 60 mm after natural aging for 50 days followed by artificial peak aging treatment with the corresponding average cooling rates in quench sensitive temperature range (420℃~230℃) increasing from 1.55℃·s-1 to 1.95℃·s-1, respectively, indicating worse hardenability and more quench sensitivity after artificial peak aging treatment. During water-spray quenching, the precipitates form firstly on grain/subgrain boundaries and then at some special positions in matrix, such as Al3Zr particles. The inhomogeneous precipitates on grain/subgrain boundaries are observed at a location of 3 mm from the quenching surface with the corresponding average cooling rate of 981℃·s-1. While inhomogeneous precipitates in matrix are observed at a location of 10 mm from the quenching surface with the corresponding average cooling rate of 37.75℃·s-1. Properties of the alloy at 25 mm from the quenching surface have little changed compared with those on the quenching surface and the corresponding average cooling rate at the location of 25 mm is 9.34℃·s-1 which is much smaller than that on the quenching surface. Therefore, during the spray quenching process, the average cooling rate of 7B50 alloy thick plates should be controlled to close to but not less than 9.34℃·s-1, in order to get excellent and uniform properties after quenching and aging.