1. A method for optimizing gray level value of ultrasonic images used in an ultrasonic system comprising: acquiring an ultrasonic image; dividing the ultrasonic image into a plurality of lines, each line comprising one or more non-evenly disposed sub-areas, wherein at least some of the non-evenly disposed sub-areas overlap at least in part; searching the non-evenly disposed sub-areas from the ultrasonic image; analyzing a change of gray level in each of the sub-areas in a direction of depth; and calculating an optimized gray level value of the ultrasonic image based on the result of the analyzing step.