A medical clamping instrument includes first and second tubular members pivotally connected together, a first rod member pivotally connected to the second tubular member, a second rod member pivotally connected to the first rod member, first and second claw arms respectively providing first and second clamping portions and first and second connecting portions and respectively pivotally connected to the second tubular member, third and fourth rod members respectively pivotally connected to the first and second connecting portions, a screw nut pivotally connected to the third and fourth rod members, a screw rod threaded into the screw nut and driven by a driving rod via the first universal joint, the fifth rod member and the second universal joint. Thus, operating the driving rod can move the first and second clamping portions, maintaining the clamping force stably.