Abstract: When it measures by using grade traverse survey in the mine, subject to the conditions, the lay of lead wires will encounter the situation of the short side inevitably. When the transition from the long side to the short side or the numerical value with a lager difference for side length back sight before the transition from short side to the long side. The operating scope of focusing mirror become augment so that the operating error of focusing mirror cause a biggish error of direction. When measuring the angle at the short side, it will cause a biggish error of angle by the impact of equipment and sighting target. By using the method of extending triangle for the short side measure, it can eliminate the impact of those errors and improve the metrical accuracy of traverse survey effectively.