The invention relates to a special unidirectional grab bucket with a size-adjustable inner wall. The grab bucket comprises a bucket body, two adjusting plates, limiting springs, six adjusting springs, six fixed hanging rings, three iron chains and bucket teeth. A row of tooth bases are evenly disposed on the front side of the lower end of the bucket body. Two second lifting lugs on the adjusting plates are connected between two pairs of first lifting lugs on the bucket body through pin shafts respectively. The upper ends and the lower ends of the adjusting plates are evenly connected with the bucket body through the limiting springs. The two adjusting plates are connected through the six adjusting springs. The six fixed hanging rings are symmetrically installed on the backs of the two adjusting plates. The three iron chains are installed among the three pairs of fixed hanging rings respectively. The bucket teeth are installed on the tooth bases of the bucket body through a pin shaft. The grab bucket solves the problem that internal space in an existing grab bucket device is limited, can achieve the function of adjusting internal space of the grab bucket, and has the advantages of being large in grabbing amount, low in labor intensity, high in work efficiency and the like.