The invention belongs to the technical field of nuclear power station construction, and particularly relates to an assembling method and a transporting method for a nuclear power station steel reactor container top sealing head. In the prior art, the construction cost of the bracket is high, the operation space in the bracket is small, and the requirement on the transporting vehicle loading ability is high when the supporting device is used to assemble the top sealing head. In order to solve the problems in the prior art, the invention discloses an assembling method and a transporting method for a nuclear power station steel reactor container top sealing head. The assembling method comprises: S1, installing an assembling bracket; and S2, assembling a top sealing head. According to the present invention, by optimizing and improving the structure of the assembling bracket in the step S1, the assembling bracket material consumption is substantially reduced so as to reduce the construction cost; and by precisely positioning the bracket fixation position, the steel plate deformation is effectively controlled, such that the requirement on the deformation during the top sealing head assembling is met, and the assembling quality of the top sealing head is ensured.