Existing calibration methods for ultra-micro-spot angle-resolved polarization scatterometers rely on rotating polarizers or liquid crystal retarders to calibrate the overall system parameters, resulting in complex operational procedures, difficulty in simultaneously decoupling multiple calibration parameters, and the introduction of additional error sources. To address these limitations, we propose an in-situ, stepwise calibration strategy for the system parameters of an ultra-micro-spot angle-resolved polarization scatterometer. Inspired by the principle of extinction ellipsometry, key system parameters — including the polarizer azimuth, ellipsometric parameters of the beam splitter in transmission and reflection, waveplate retardation and azimuth, ellipsometric parameters of the polarization beam splitter in transmission and reflection, and the orthogonally polarized transmittance of the objective lens — are sequentially calibrated with high precision. The proposed method was applied to calibrate a self-developed ultra-micro-spot angle-resolved polarization scatterometer, which was subsequently used to measure standard SiO₂ thin films and rectangular grating samples. The results demonstrate that, after in-situ calibration, the instrument achieved a film-thickness measurement repeatability of 0.1 nm, and the extracted grating morphology parameters exhibited excellent agreement with standard values, thereby verifying the effectiveness of the proposed calibration method.