Acoustic measurement of seabed backscatter is very effective due to its ability to operate remotely and continuously. A quantitative or scientific echosounder (QES) has been developed to measure fish backscatter for estimating fish abundance, but it can also be readily applied to measure backscatter from other marine or underwater objects. This paper describes a method for measuring seabed backscatter using the QES. First, we present a technique developed by the author’s group with some reinforcement particularly in error estimation. Next, we develop a new theoretical model applicable to sloping seabeds and inclined acoustic beams. The analysis shows that, by using the average volume backscattering measurement function (echo integration), we can accurately measure seabed backscatter, even on the sloping seabeds or during transduce motion. Moreover, for a largely oblique beam, such as 45°, backscatter can be measured with appropriate correction, and the results can be used to calibrate a multi-beam echosounder.