1. A method for detecting displacement in elastography within an ultrasound imaging system, comprising: acquiring at least two successive frame images from ultrasound signals using an ultrasound probe; selecting a target point in a first frame image; obtaining a cross-correlation phase calculation location of the target point in a second frame image; calculating a cross-correlation phase based on the cross-correlation phase calculation location; and calculating a longitudinal displacement based on the cross-correlation phase; wherein the cross-correlation phase calculation location comprises a longitudinal location for cross-correlation phase calculation; and wherein obtaining a cross-correlation phase calculation location of the target point in a second frame image comprises: determining a longitudinal displacement of a previous calculation point of the target point; in the second frame image, searching a point which has maximum correlation with kernel data within a region which centers on a sum of location of the target point and the longitudinal displacement of the previous calculation point, wherein longitudinal location of the point which has maximum correlation with kernel data serves as the longitudinal location for the cross-correlation phase calculation of the target point in the second frame image.