For the high precision strapdown inertial navigation system in north, the rotary modulation will not get rid of the inner-arm error.In the north-seeking process, because of the rotating device,it will introduce the error into the north navigation calculation. In this paper, first, the error of the within arm is modeled and analyzed, and the mathematical expression of the error effect is deduced. And two factors affecting the error of the within arm are analyzed and determined, then error compensation method of the within arm is put forward. Finally the simulation results are verified.