A new method for rotation mode platform inertial navigation initial alignment is presented. This method is based on sinusoidal varying speed rotation mode and thus makes the factors decoupling in real time. In this way it makes the estimation process much faster and more precise compared with previous methods. The error equations of a rotating platform inertial navigation system are introduced. We give detailed description and analysis of this method. Simulation is done based on this new method. Analysis and simulation results indicate that this method makes the platform system initial alignment achieved desired results in 207.5 seconds, while the traditional methods takes 388.1 seconds. The accuracy also has highly improved. The standard deviation of gyroscope drift decreased from 0.03°/h to 0.005°/h. This provides a theoretical foundation for platform inertial navigation system rotating mode during actual experiment and application.