Inertial navigation systems often operate in complex mechanical environments. Under conditions of high-speed motion or severe vibration, the output of accelerometers can be affected by the base angular velocity, leading to measurement errors. Therefore, this paper investigates the formation mechanism and dynamic characteristics of the output error of the Pendulous Integrating Gyroscopic Accelerometer (PIGA) under base angular motion. First, starting from the dynamic modeling of the PIGA, a comprehensive error model incorporating the combined effects of linear acceleration, angular velocity, and angular acceleration is established. Then, based on the simultaneous consideration of three-axis angular velocity and angular acceleration inputs, the error expression of the PIGA is derived. The generation mechanism, magnitude, and coupling relationship with system parameters of error terms related to angular motion are analyzed, revealing the key error components that significantly impact output accuracy and require compensation. Finally, based on the dynamic simulation model, the influence of base angular motion inputs on the dynamic response of the PIGA output is studied. The results show that under different combinations of angular velocity and angular acceleration inputs, the error characteristics of the PIGA output exhibit significant differences. Therefore, the effect of dynamic base angular velocity on the PIGA output is substantial and should be addressed and compensated for in practical engineering applications.