The utility model discloses a built -in steam stabiliser, arrange with pressure vessel's upper cover integration, it includes the baffle, electric heating pipe and the equipment of spraying, the baffle sets up and becomes cooling agent space and stabiliser space in the bottom of upper cover with the internal partitioning with pressure vessel, a plurality of intercommunication cooling agent spaces have been seted up on the baffle, the undulant hole in stabiliser space, electric heating pipe locates the top of baffle and lies in the stabiliser space, the equipment of spraying is located the top of upper cover and is fed through a return circuit cooling agent system. Arrange stabiliser and upper cover integration simple structure is compact, it is convenient to make, reduce cost, and come the undulant tubular construction of replacement through undulant hole, and eliminated the possibility that undulant pipe takes place the cut, reinforceed the flow fluctuation between reactor and the stabiliser, simultaneously, the stabiliser separates the upper cover with reactor coolant, has reduced the thermal stress of upper cover flange, and this built -in steam stabiliser has higher economic nature and higher security.