Traditional attitude algorithms of Strapdown inertial navigation system often use incremental angular signals. However, the outputs of interferometer FOG are angular rate signals, it will cause precision restriction and much larger algorithm error by extracting the angular increment. This paper presents a new two-loop iteration algorithm only based on gyro rate input, new algorithms were optimized by minimizing algorithm drift error under classical coning motion, and the coning compensation coefficient equations and algorithm error expressions were deduced．Simulation result shows that the improved new algorithm was greatly improved in accuracy than regular angular mate algorithm with the same number of gyro angular rate samples．The introduction of the new algorithm is a new idea to compensation coning error of strap down inertial navigation(SINS)system with FOG.