Static method of slope stability analysis is simple and play a distinctive role in slope seismic stability. Sliding mass is traditionally divided by vertical slice. Errors should be induced in calculating horizontal seismic inertial force and anti-sliding moment according to vertical slice. Considering horizontal seismic inertial force in foundation
safety factor is uncertain. The cause of errors and uncertain is analyzed
and an improved method is put forward. Calculating sample shows the improved method is reasonable.