Inertial Navigation System (INS) is a fully autonomous navigation technology that offers several advantages, including high short-term accuracy, comprehensive output data, and enhanced concealability. However, due to the integration processes involved in its calculations, the system’s errors tend to accumulate and diverge over time in the absence of external corrections, leading to a significant degradation in long-term accuracy. Therefore, analyzing the sources of INS errors and understanding their propagation patterns is crucial for improving the system’s positioning accuracy, and this analysis also serves as a fundamental theoretical basis for constructing integrated navigation models based on inertial systems. This chapter begins by introducing the typical sources and modeling methods of sensor errors, system errors, and navigation errors in an INS. It then analyzes the error propagation model of pure inertial navigation under stationary conditions, revealing the mechanisms by which different error sources impact navigation accuracy. Subsequently, introduce the commonly used error analysis method for inertial devices—the Allan variance method, which analyzes the primary sources of random errors, and, using the gyroscope as an example, correlates specific error components with Allan variance parameters. Finally, accuracy improvement measures for inertial navigation systems are discussed, including calibration, temperature compensation, and rotary modulation, along with a rationale for the underlying improvement mechanisms. The goal of this chapter is to help readers understand the mechanisms of INS errors and the common techniques for error suppression, enabling better application of the system and maximizing its inherent advantages.