Dispersion and attenuation are important attributes of seismic data that can provide important information about reservoir rock lithology, pore fluid type, and pore structure. Based on Cheng’s pore-aspect-ratio spectrum inversion methodology, we related the closure and deformation of soft pores to the measured pressure-dependent porosity data. With this additional constraint, the inverted pore-aspect-ratio spectrum and concentrations are more realistic. The complex pore structure controls two important intrinsic dispersion and attenuation mechanisms: Biot flow and squirt flow. We modified and extended Tang’s unified velocity dispersion and attenuation model and made it applicable to poroelastic media with a complex pore structure under the undrained condition. The inverted pore-aspect-ratio spectra from pressure-dependent ultrasonic velocity measurements were put into the modified Tang’s model to predict velocity dispersion and attenuation in full frequency range at various differential pressure conditions.