We present a damping modification factor (DMF) model for the total acceleration spectrum from subduction slab earthquakes. The model can be used for scaling a 5% damped design spectrum not associated with a particular earthquake that occurred in a subduction slab. The DMF model uses site‐period‐based site classes as the site‐effect proxy. DMF models were constructed based on the spectrum for 13 damping ratios and 34 spectral periods; the DMF values can be calculated for any damping ratio between 1% and 30% and at any spectral period between 0.03 and 5.0 s. At moderately long and long spectral periods, the DMF values for acceleration spectrum are similar to or less than those for the displacement spectrum for a damping ratio of less than 5%, whereas the DMF values for the acceleration spectrum are similar to or larger than those for the displacement spectrum when the damping ratio is more than 5%. The standard deviations for acceleration and displacement spectra are similar at short or moderately short spectral periods, but those for the acceleration spectrum are about twice those for the displacement spectrum at long spectral periods. All standard deviations decrease linearly with increasing damping ratios in the logarithm scale when the damping ratio is less than 5% and increase linearly with increasing damping ratios in a logarithm scale for the other damping ratios. A set of simple functions for calculating various standard deviations is presented. The spectra from the Zhao, Jiang, et al. (2016) study for slab events scaled by the DMF values for other damping ratios vary smoothly with spectral period and have a trough at short spectral periods for a large event, a short distance, and high damping ratios. The relatively large between‐event and within‐site standard deviations are from the source and path effects.