A novel alignment method for swaying base, such as a sailing ship, is presented with the aid of a cascade of FIR filters and a transitional inertial frame. For the method, an investigation was made about the error characteristics, which shows that the alignment accuracy mainly depends upon the equivalent biases of gyroscopes in east direction and performance of FIR filter group, the aligning time rests with the orders of the FIR filter group, and the problem of large initial misalignment is resolved in theory. Results of experiments show that the proposed method converges much faster than the traditional methods at no cost of precision and also works well under any large initial misalignment.