Due to the velocity dispersion error, the laboratory ultrasonic velocity measurement of core samples cannot be directly used to calibrate the acoustic well logging velocity of reservoir in Southwest Weizhou depression. The further work of data processing and analysis for the ultrasonic velocity dispersion were conducted for 5 representative sandstone samples of W3 reservoir. By comparing the compressive velocities of sandstone samples saturated with the brine and four oils of different densities, the method and process of velocity dispersion mechanism analysis were greatly improved. The assumption was proposed and verified that the modulus of non-relaxation wet solid frame was equivalent with the one of dry frame under high pressure condition when the sandstone samples were ruled by squirt flow mechanism. The estimation method of R value, which was the critical squirt length of BISQ theory, was also proposed and verified. Then, the method to quantitatively predict the P-wave velocity dispersion of W3 reservoir sandstones in full waveband was accomplished. Finally it is concluded that the mechanism and degree of velocity dispersion depend on both the physical properties (porosity and permeability) of samples and the kinematic viscosity of pore fluids. Using the velocity dispersion prediction formula for full waveband, the ultrasonic velocity measured in laboratory (high frequency band) can be corrected to different frequency, and then meet the demands of rock physics analysis for well logging (medium frequency band) and exploration seismology (low frequency band). Thus, the complete technique for velocity dispersion analysis and prediction has certain practical meaning.