A protractor comprising an angle scale provided with a dial gauge with a circle center, one end of the bottom edge being connected with a first side edge, and the first side edge and the bottom edge forming an included angle, and the angle of the included angle being 135 degrees; a right angle rocker arm, comprising a reading arm part and a tail arm part forming a right angle with the reading arm part, the right angle rocker arm rotating relative to the angel scale such that a first edge of the tail arm part and the bottom edge form a first change included angle, the value of the first change included angle being read by means of the reading arm part and the dial gauge; and a locking structure for rotationally connecting the angle scale and the right angle rocker arm.