The invention relates to an image gray scale distribution statistical device and method thereof. The image gray scale distribution statistical device comprises a comparing unit, a counting unit, a memory unit and a data distribution unit; wherein the comparing unit is used for judging gray scale information of mass data of the input image and then outputting corresponding counting signal; the counting unit is provided with a plurality of counters, and each counter is corresponding to preset gray scale information with a count value and updates the count value according to corresponding counting signal; when the count value is more than default value, the counting unit outputs a pulse signal of the corresponding gray scale information; and the data distribution unit updates image gray scaledistribution stored in the memory unit according to the pulse signal. Thus, buffers required by the counter can be reduced and hardware cost can be reduced.