In order to reveal the water absorption characteristics of recycled aggregate of old concrete slabs, the granulated petrochemical recycled coarse aggregate was classified according to the particle size and mortar content. Firstly, the water absorption regularity and influencing factors of granulated petrochemical recycled aggregate particles were discussed from the point of view of grain granules, and the reason was analyzed by using the scanning electron microscopy. Then, according to the quantitative relationship between granulated petrochemical recycled aggregate and granulated petrochemical recycled coarse aggregate, the water absorption property and influencing factors of granulated petrochemical recycled coarse aggregate were further discussed by means of mathematical deduction. The results show that the mortar centent and the gradation have important effects on the water absorption of granulated petrochemical recycled aggregate. Because the mortar content makes the recycled aggregate surface complex and of porous structure, the recycled aggregate has unique water absorption characteristics. Considering that the crushing method affects the degree of crushing of the old cement concrete and the degree of mortar handling, the importance of the crushing technological process was also illustrated.