The invention discloses an ultralow-temperature top-entry ball valve. The ultralow-temperature top-entry ball valve comprises a valve ball, valve seats, a valve body and pressing rings. Matched unlocking structures are formed on each valve seat and the corresponding pressing ring in the axial direction, and by rotating the valve seats in the circumferential direction, the unlocking structures are suitable for driving the valve seats to stop sealing the valve ball till the valve ball can be taken out. According to the ball valve, by means of the unlocking structures on the valve seats and the pressing rings, the valve seats are pushed to move outwards in the axial direction of the valve body till the valve ball can be taken out of the space between the two valve seats, the unlocking structures reach the limit distances at the moment and are locked, and then a maintainer can take out the valve ball and internal parts of the valve; after maintenance is completed, the maintainer just needs to rotate the valve seats backwards or continue to rotate the valve seats, then the valve seats move close to the valve ball till the valve ball is sealed, and external power is not need; and operation is easy, convenient and fast, and online rapid maintenance of the ball valve is achieved.