Abstract: C4.5 algorithm is a classic decision-tree-based data-mining algorithm, It classifies and analyzes data based on information entropy theory. It mainly includes such following steps as pre-procession of data, creation of decision-tree, pruning of decision-tree, rules extraction from decision-tree. C4.5 algorithm is introduced in this paper, and it is applied in a data analysis on second stage investigation of forestry resources. The promising prospect of data mining in investigation and data analysis of forestry resources has been proved.