1. A method of calibrating ultrasound velocity to obtain a calibrated ultrasound velocity for imaging, the method comprising 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 from a plurality of sub-apertures of a probe to generate a plurality of aperture images; (c) performing an MAE (Mean Absolute Error) process on the aperture images to obtain MAE values, obtaining a velocity and two sub-aperture images corresponding to an MAE value located within an MAE range; (d) finding a first sub-aperture and a second sub-aperture corresponding to the two sub-aperture images, performing the beam-forming process on the ultrasound non-delayed data set with the 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) performing the MAE process on the first aperture image and the second aperture image to generate an image error corresponding 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 MAE value.