The seismic earth pressure behind a flexible retaining wall is closely related to the wall displacement mode and displacement magnitude, whereas existing theories for active seismic earth pressure often neglect the actual stress-deformation characteristics of flexible retaining structures. To clarify the variation of non-limit state seismic earth pressure for limited soil behind flexible retaining walls and to address the insufficient consideration of wall displacement effects and seismic temporal effects in existing seismic earth pressure calculations, analytical solutions for the distribution of non-limit state seismic earth pressure and the resultant seismic earth pressure were derived based on the horizontal differential element method and the pseudo-dynamic method. The results show that the seismic earth pressure for limited soil behind flexible retaining walls exhibits periodic characteristics with time and nonlinear characteristics with depth, and that the calculated results agree well with the shaking table test results. The seismic earth pressure decreases with increasing initial internal friction angle, increasing average displacement area ratio, and decreasing width-to-height ratio of the limited soil. Seismic acceleration mainly affects the amplitude of the resultant seismic earth pressure. The height of the application point of the resultant seismic earth pressure increases with increasing internal friction angle and average displacement area ratio. The results can provide a theoretical reference for the seismic design of flexible retaining structures in practical engineering.