A string clamping device of a threading machine for a racket contains: a clamp unit, a tightening unit, an adjustment disc, a connection column, and a fixing structure. The clamp unit includes a body, a locking seat, and two first springs. The body has multiple first paws, the locking seat has multiple second paws, and among the multiple first and second paws is defined a gap of the clamp unit. The tightening unit includes an abutting block and an operation handle, the body has a first protrusion with a first through orifice, and the adjustment disc includes multiple teeth, wherein between any two adjacent teeth is defined a recess. The fixing structure includes an affix member, a coupling shaft, a second spring, a screwing element, and a support part, wherein the affix member has a retaining portion, and the second spring contacts with a drive tab of the support part.