Free phosphoric acid in the diatomite-phosphate solid acid catalysts was characterized by means of ion chromatography (IC), Hammett pH indicators and chemical titration. The results show that the free phosphoric acid is composed of 5 similar to 35 wt.% H3PO4, 1 similar to7 wt.% H4P2O7 and < 1wt.% polyphosphoric acid, and its composition determines the acid strength distribution of the catalyst. When the total amount of free phosphoric acid containing >3 wt.% H4P2O7 is within 9 similar to 26 wt.% (in P2O5), the catalyst shows higher catalytic activity for propylene oligomerization with propylene conversion >75wt.%. Besides, free phosphoric acid has an important effect on the catalytic selectivity. The relationship between them is discussed.