The invention discloses a ball valve, in particular to an ultralow temperature top-assembled fixed ball valve, and provides the ultralow temperature top-assembled fixed ball valve capable of setting pressure relief direction during mounting. The ball valve comprises a valve body, wherein the valve body comprises a valve cavity and two connection parts at the two ends of the valve cavity; the two connection parts are named as a first connection part and a second connection part respectively; the end, towards the valve cavity, of the first connection part is provided with a one-way sealing valve seat structure; the end, towards the valve cavity, of the second connection part is provided with a bidirectional sealing valve seat structure; an elastic seat is arranged on the second connection part; a matching section of the bidirectional sealing valve seat structure is inserted into the second connection part; a piston ring is positioned between an outer ring of the matching section and an inner ring of the second connection part and can axially move; the inner ring of the second connection part is provided with a limiting ring which can be tightly attached to and matched with the piston ring; the axial movement of the piston ring is limited between a matching surface of a valve cavity section of a supporting ring and the limiting ring.