In this paper, for a seismic analysis of an offshore subsea manifold, Response Spectrum Analysis(RSA) and Time History Analysis(THA) were conducted under a various analysis conditions. Response spectrum and seismic design procedure have followed ISO19901-2 code. In case of THA, The response spectrum were converted into artificial earthquake history and both of Explicit and Implicit solvers were used to examine the characteristics of seismic analysis. For the verification, Various seismic analysis methods were applied on a single degree of freedom beam model and a simplified model of the actual manifold. The difference between the results of RSA and THA on the simplified manyfold model evaluated for the analysis of the actual manifold. Because THA is impossible in case of real complex structure such as a manifold, Safety of the actual manifold structure was accessed by using the RSA and the difference between the results of RSA and THA from the simplified model.