Objective To study the correlation between the amount of reduction loss and the length of fixator moment arm after treating acrmioclavicular（AC）joint dislocation with the endobutton technique as viewed from mechanics. Methods 24 patients with acute AC joint dislocation were treated with the endobutton technique and made the follow-up from Mar. 2009 to Dec. 2010 in Shanghai Ninth People’s Hospital. The loss of reduction was observed by evaluating the change of distance between the coracoid and clavicle three days and six months after the operation, respectively. Meanwhile, the length from the upper endnobutton midpoint to the proximal end of clavicle (L1) and the whole length of clavicle (L) were measured and the ratio of L1/L as the moment arm of the fixator was calculated to study the correlation between reduction loss and moment arm. Results All the 24 patients received good reduction after the operation, and the distance between the coracoid and clavicle returned to normal. The average distance between the coracoid and clavicle three days and six months after the operation was (28.2±3.9) and (29.5±4.1) mm, respectively. The amount of average reduction loss was (1.3±1.2) mm. The average moment arm was 0.79±0.03. There was a significant negative correlation between the reduction loss and moment arm with the correlation coefficient -0.498(P＜0.05). Conclusions For the treatment of AC joint dislocation using endobutton technique, the position of the endobutton is closely related with the loss of reduction postoperatively. The shorter the moment arm, the greater the loss of reduction would be. The proper moment arm should be around 0.80.