Abstract: layer velocity calculated with velocity spectrum is in fact intrabed root－mean－square velocity due to the restriction of seismic resolution．Thus the average velocity computed based on these results is larger than its actual value and is higher than VSP average velocity． The lower the seismic resolution is the higher the error of the average velocity derived from velocity spectrum． Omission of stacking velocity during interpretation of velocity spectrum can also result in a higher average velocity． Enhancing seismic resolutios is the base for improving the calculation accuracy of velocity． Utilization of velocity and careful interpretation of velocity spectrum are the ways to improve velocity calculation accuracy．