The invention discloses a measuring system for determining the internal boundary of an underground karst cave and a use method of the measuring system. The measuring system comprises a mechanical contact arm, a rotating device, a sensing device, a telescopic rod, an anti-touch device and a GPS positioning device; the mechanical contact arm is an extensible thin rod; the contact arm cannot rotate,and can only extend and contract in a horizontal plane; the rotating device is used for driving the contact arm to rotate; the sensing device is located at the outer edge of the contact arm; when thecontact arm extends to a certain length to touch the inner wall of the cave, the sensing device automatically records the length and azimuth of the contact arm, controls the contact arm to contract toa starting position and transmits data back to a host; the telescopic rod is located at the upper part of the measuring system and is used for controlling the rise and fall of the measuring system; the anti-touch device is located at the bottommost end of the measuring system; when the anti-touch device touches the bottom of the cave, the telescopic rod stops extending, and the measuring system no longer descends; and the GPS positioning device is located inside the measuring system.