Three-ball fisher is a special fishing tool which fishing sucker rod coupling or other shoulder of sucker rod in casing.

Three-ball fisher according to the position change of three balls in slant borehole to change the diameter size of public inscribed circle, thus the shoulder of sucker rod and coupling was allowed to pass. After the sucker rod with coupling or shoulder entering guide shoe, the coupling or shoulder push the steel ball rising along slanted hole. The inscribed circle is formed through three balls increase gradually. After coupling and shoulder passed three balls, three balls according to his dead weight fall after rise along slanted hole, near to the body of sucker rod.

Bring up drilling tool, the shoulder of sucker rod or the size of coupling is larger so that can not pass to press three balls, the three balls in slanted hole under the action of slanted hole, applying a clamp force in radial direction to the fish, thereby the fish is cached.