In severe defocused cases during stellar facula imaging， traditional gradient evaluation algorithms often present weak gradient details and fail to correctly identify the direction of focus. Here， we designed an automatic focusing system for the evaluation of star imaging characteristics to realize the rapid imaging and reliable tracking of stars based on a star-following system； moreover， we studied the focusing evaluation algorithm， automatic focusing structure， and automatic control of the system. Differences in the focus and focus gradient of a facula image were analyzed based on the grayscale distribution of the star system images. The background value was included in the evaluation contribution using the characteristics of grayscale differences in stellar facula imaging， and the grayscale difference contribution of the facula signal was added to the traditional gradient evaluation function. To develop the automatic focusing system， three steps were followed： design of the focusing hardware and system structure adapted to the improved algorithm， construction of the focus assembly using a stepper motor and focus base， and the use of a calculation control unit to drive the focus assembly. Experiments and analyses were conducted using a star-tracking device equipped with the focusing system. The experimental results revealed that the autofocus device could focus steadily on the star-tracking system. The improved algorithm presented better focus orientation than the traditional gradient algorithm. The local extreme point was reduced to zero while maintaining the 67% sensitivity of the traditional gradient algorithm. Thus， the improved algorithm was not affected by weak gradient details. It could maintain better focus directionality and focus sensitivity at different defocus levels. Equipped with an autofocusing ability， the developed star-tracking system can rapidly determine a suitable focal distance and assess atmospheric optical characteristics.