More than 30,000 inscribed bamboo slips and wooden tablets of Han times were discovered and unearthed in Juyan 居延 (nowadays Ejina 額濟納 Banner, Inner Mongolia Autonomous Region) in the last century. By examining these slips, especially the Ejina Han jian 額濟納漢簡 unearthed in 1999, the author discusses the duties and routine work of the shili 士吏, who were the officers serving in a company (hou 候) in the Juyan frontier. As indicated in the bamboo slips, officers with the rank of sub two-hundred bushels (shi 石) and the monthly salary of 1,200 coins (qian 錢) were subordinated to the head of the Jiaqu 甲渠 company. However, they were usually dispatched to the fortress towers (hou 候) and beacon towers (sui 燧) to supervise the work of the watchtowers and to command the defense of the scouting areas from the attack of the northern nomads, the Xiongnu 匈奴 in Han China. These officers were responsible for watching intruders, managing the signals (烽火 fenghuo) on the fortress towers and guarding against possible robbers. In addition to these routine work, there were other duties. The “Seven Instructions to the Officers Shili,” a newly excavated wooden document from the Ejina Han jian, yielded valuable information on the duties and obligations of these officers. In brief, they inspected the implementation of government policy and took charge of the management of the work of their fellow officials, such as the hou zhang 候長 (heads of the fortress towers), hou shi 候史 (the clerks of the fortress towers), and sui zhang 隧長 (heads of the beacon towers). Furthermore, these officers shili were expected to examine armament supplies and the allocation of army provisions, to assess the performance and to give suggestions for promotion and dismissal to their colleagues. Since they also performed as the commanding officers in the Juyan frontline, they routinely fought against the Xiongnu cavalry with Han garrisons and conscripts as well.