Dynamic equations of pendulums in strong motion instrument are analyzed. The equations imply horizontal recordings may be significantly influenced by ground surface tilt while the effects can be ignored for vertical recordings. The difference causes anomaly of horizontal to vertical Fourier amplitude spectra ratio, which is used to identify affected low frequency range.. The low frequency filter based on spectral ratio was employed to estimate the coseismic ground surface tilts in WenchuanMs8.0 earthquake. Results show the ground surface tilts are generally less than 1 degree and mainly happened within 30 kilometers to fault surface rupture traces, while rarely discovered in area outside 100 km and squared horizontal peak accelerations less than 200 cm/s2. Comparison of ground surface tilts under spectral ratio being 2 and 4, shows ground surface tilting amplitudes usually increased when spectral ratio is decreased, but residual ground surface tilts have small change.