A new initial alignment method was proposed to meet the requirements of precision and efficiency on inertial navigation system (INS). The principle of initial alignment was studied; A new attitude error model was introduced to resolve the problem of large and accumulated heading error; New attitude matrix models were deduced, and leveling and heading attitude errors were processed step-by-step; Leveling attitude was used directly and indirectly in recursive operation to avoid the heading angle error effect on the leveling attitude alignment and improve the heading alignment. Finally, the simulation platform of INS based on error separation was built.