Quality factor (Q) is a key parameter for geophysical exploration in that it indicates the magnitude of sediment attenuation.When attenuation is large,quality factor computed by conventional methods such as spectral ratio method is smaller than 2π,which is the low limit of Q calculated by defining expression.Conversion formula for the two quality factors is introduced by investigating the differences between them.Model tests indicate that,when quality factor is lower than 30,relative error between quality factors calculated by two methods is larger than 10% so that they should be distinguished one from another.Based on this study,the authors put forward some advises.First,during Q-V fitting process,it is prefer to use the quality factors calculated by defining expression.Second,conventional method is preferred in gaining quality factors in reverse Q compensation process.Third,when the comparison of quality factors is needed in an oil field,quality factors should be converted to the same class before the comparison.