Landslide force history inverted from long-period seismic records for a landslide has been used to extract its physical parameters. An important precondition is that the inverted landslide force histories are reasonable approximations. We first discuss how to estimate the accuracy of the inverted force histories and then propose an approach to determine the proper frequency band used in the inversion. We perform a long-period seismic waveform inversion for the 2003 Qianjiangping landslide to obtain its force-time history and estimate its movement parameters. Based on the results, we build a simple model to study the effects of the portion of the initial sliding mass and the location of the mass entrainment on the seismically estimated mass, which shows that if the entrainment occurs at the slow-down phase the seismically estimated mass is a closer approximation of the initial sliding mass.