1. A method of calibrating ultrasound velocity to obtain a calibrated ultrasound velocity for imaging, the method comprising the following steps: (a) receiving an ultrasound non-delayed data set; (b) performing a beam-forming process on the ultrasound non-delayed data set with a plurality of velocities defining a plurality of sub-apertures of a probe to generate a plurality of sub-aperture images; (c) performing a Mean Absolute Percentage Error (MAPE) process on the sub-aperture images to obtain MAPE values defining a MAPE range, obtaining an intermediate calibration velocity and two calibration sub-aperture images corresponding to a pre-selected MAPE value located within said MAPE range; (d) selecting a first sub-aperture and a second sub-aperture corresponding to the two calibration sub-aperture images, performing the beam-forming process on the ultrasound non-delayed data set with the intermediate calibration velocity to generate a first aperture image corresponding to the first sub-aperture and a second aperture image corresponding to the second sub-aperture; (e) utilizing the first aperture image and the second aperture image to generate an image error figure having an error curve; (f) finding a trend curve according to the error curve; and (g) finding a lowest point on the trend curve and finding a velocity corresponding to the lowest point to obtain the calibrated ultrasound velocity, wherein the lowest point on the trend curve shows the lowest MAPE value.