<p>Aerostatic spindles are extensively employed in ultra-precision machining owing to their superior rotational accuracy, negligible wear, and low thermal distortion. However, during practical machining operations, the combined gravitational effects of the workpiece and fixture inevitably induce journal misalignment, which alters the air film pressure distribution and degrades spindle rotational accuracy, particularly under variable external load conditions. In this study, a comprehensive dynamic model of an aerostatic spindle is developed by incorporating journal misalignment, nonlinear air film forces, rotor unbalance, and alternating sinusoidal loads. The transient compressible Reynolds equation is coupled with the rotor dynamic equations, and the time-varying spindle axis trajectory is numerically solved using an iterative Euler scheme. The influences of journal misalignment angle, rotational speed, rotor mass eccentricity, and alternating load amplitude and frequency on spindle rotational accuracy are systematically investigated through time-domain trajectory analysis and statistical indicators. The results demonstrate that journal misalignment leads to a pronounced axial non-uniformity in air film pressure and a reduction in throttle orifice outlet pressure, thereby significantly increasing spindle vibration amplitudes. Under misalignment conditions, increases in rotational speed and rotor mass eccentricity further amplify vibration responses and deteriorate rotational accuracy. When subjected to alternating sinusoidal loads, the spindle exhibits enhanced vibration amplitudes and trajectory distortion, while non-synchronous excitation frequencies induce irregular axis trajectories and reduced dynamic stability. The proposed model provides a quantitative and efficient approach for predicting spindle rotational accuracy under realistic machining conditions, offering valuable guidance for the design, load evaluation, and performance optimization of ultra-precision aerostatic spindle systems.</p>