The current cofocus maintenance strategy of the primary mirror of the Large Sky Area Multi-Object Spectroscopic Telescope (Mb) using Shack–Hartmann Wavefront Sensor (SHWFS) interrupts the observation mission. This study proposed an Eddy current edge sensor with high precision and accuracy to achieve closed-loop cofocus control. Through structural design, interaction matrix deduction, installation parameter analysis, and performance calibration, the edge sensor was validated for use in detecting segment attitudes. The minimum cofocus maintenance system comprising three segments was separated from Mb and established to validate the effectiveness of the closed-loop cofocus control with the novel edge sensor. The closed-loop control system was effective within less than 1.5°C environmental temperature changes after 13 h, which is considerably longer than the maintenance time with the SHWFS. Moreover, continuous observation without interruption is possible during one observation night.