A sub-picture image decoder is disclosed, which essentially uses a sub-picture buffering unit to temporarily store at least one sub-picture data, an adjustment unit to store plural types of adjusting parameters, a control unit to compare the sub-picture data with the sub-picture display formats stored in a look-up table. The control unit receives adjusting signal generated by an input unit to retrieve corresponding adjusting parameter and adjust the corresponding sub-picture image for display according to the adjusting parameter.