An automatic-leveling system is developed.Three mechanical supportings are chosen to sustain a large-size photoelectric equipment,and a obliquity-sensor fixed on the pedestal of the photoelectric equipment is used to measure the size and direction of the obliquity angle from a baseplane. Based on a leveling algorithm,the obliquity angle is converted to elongations of the mechanical supporting to drive the mechanical supporting to level the baseplane. The reasons for choosing three supporting points and mechanical supporting are introduced,then mechanical supportings and the entire transmission system are designed.A driven circuit and automatic-leveling arithmetics are given also. Automatic-leveling experiment indicates that the leveling precision of the automatic-leveling system reach 0.003°,and the leveling time of photoelectric equipment is about 120 s,which shows that this system is better than formerly systems on prepared time and precision.