The zeolite Y synthesized by in-situ crystallization technique was modified with phosphate. The crystalline structure and surface acidity of the modified zeolite Y(named zeolite PY) were characterized by using techniques such as X-ray diffraction ( XRD) , NH3 temperature programming desorption (NH3-TPD). The results showed that the introduction of phosphorus had no effect on crystal form of the zeolite PY, and the caused impact on relative crystallinity was obscure. Moreover, the space unit shrank, the cell parameter decreased, and the molar ratio of Si/Al increased. It was found that the modified PY catalyst could effectively adjust the total acid amount and acid strength of the catalysts. The introduction of phosphorus could also decrease the total acidity, weak acidity and strong acidity, but increased the moderately strong acidity. Further evaluation that carried on a small fixed fluidized bed showed that the modified PY catalyst exhibited excellent heavy oil cracking capability an d superior selectivity to desired products．