Compared with the platformbased inertial navigation system,the strapdown inertial navigation system has the advantages of small size,low cost,fast startup time and high reliability.But,its performance will be influenced by the angular motion of the carrier. Combining the advantages of the strapdown inertial navigation system and platformbased inertial navigation system,a compound inertial navigation system is investigated in this paper.Firstly,the principle of the proposed system is described in detail. Secondly,the error equations of the proposed system are derived.It is analyzed theoretically that the performance of the proposed system won′t be influenced by the angular motion of the carrier.The effectiveness of the method is verified by the simulation experiment.