The utility model provides a pair of a direct -reading calliper for measuring channel external diameter, including stationary dog and movable claw, the stationary dog is fixed on the measuring tool body, and movable claw movable mounting is fixed with the handle in volume specific upper end on the measuring tool body, set up the back -moving spring between handle and movable claw, sets up an angle sensor at the movable claw with volume specific articulated department, sets up the digital display box on the movable claw, and the digital display box links to each other with angle sensor. The beneficial effects of the utility model are that: through set up digital display box and angle sensor on the movable claw, set up the extension spring between two plier leg, utilize the angle of angle sensor measuring, transmission signal to treater shows the meticulous measurement of being convenient for like this through the digital display box, specifically replace the position quantity of the needs of traditional calliper transposition, in the length of having reduced calliper for whole calliper portable.