To accurately estimate the saccharifying activity of crude cellulase preparations for insoluble cellulosics, a practical approach was designed. The interferences by changes in hydrolysis time and concentration of cellulase and cellulosics on the assay could be overcome by selection of the specific conversion percentage of cellulose as a function of cellulase concentration, that is, the hydrolysis percentage of filter paper by unit cellulase per minute as the objective function with respect to different concentration of crude cellulase. A rational and governing equation for crude cellulase assay was derived, and reliable results for quantitatively estimating the amounts of crude cellulase during hydrolysis progress of several cellulosics were obtained.