The accuracy of downhole drilling attitude relies on the error calibration of the inertial measurement unit ( IMU). However, ellipsoid and plane fitting based on simplified linear models have failed to meet engineering requirements. The periodic residual error of axial tilt angle and tool face angle needs further compensation, and its nonlinearity is confirmed by multi-point tumble tests and Monte Carlo simulations. In this study, we employ post-fit identification through tilt tumble tests and perform parameter optimization, followed by proposing an equidistant balance correction technique. Data is collected under typical cases where ellipsoid fitting, plane fitting, tumble optimization, and equidistant balance correction techniques are applied in experiments. The results indicate that tumble optimization and the equidistant balance correction technique effectively reduce the nonlinearity of attitude error, reducing the nearvertical angle error of axial tilt from an initial ±0. 1° to calibrated ±0. 000 8° (1σ), and the tool face angle error from intial ±1° to ±0. 016 8° (1σ).