The invention relates to a clamping box the dimension of which can be adjusted by sections and the positioning piece thereof; wherein, the positioning piece is used for fixing the position of the bearing side plate in the clamping box and comprises a plurality of first slotted holes and a plurality of second slotted holes. The first slotted holes are arranged at one side of the positioning piece; wherein, the adjacent first slotted holes one by one have the spacing with fixed length, thus being used for adjusting of bearing side plates by sections. The second slotted holes are arranged at the other side of the positioning piece; wherein, the second slotted holes and the first slotted holes face each other and are staggered; furthermore, the spacing between two adjacent second slotted holes is equal to the fixed length, thus being used for the adjusting of the bearing side plates by sections. The clamping box the dimension of which can be adjusted by sections comprises two bearing side plates and a plurality of positioning pieces. By the clamping box the dimension of which can be adjusted by sections and the positioning pieces, the invention can save manufacture expense and time and leads the clamping box to be adjusted at any moment and to bear the baseplate with various dimensions.