From English Wikipedia:
The flag of Tibet was reintroduced in 1912 by the 13th Dalai Lama, who united the army flags of various provinces to design the present one. Since then, it served as the all-Tibet military flag until 1950[1]. It remains the emblem of the Central Tibetan Administration headquartered in Dharamsala, India. As a symbol of the Tibetan independence movement, it is banned [1] in the People’s Republic of China, including the Tibet Autonomous Region which corresponds to the former area of control of the Tibetan government at Lhasa, as well as other areas in greater Tibet.
During the era between 9th and 18th century, Tibet did not have an official army. In late 18th century, after Qing government defended Tibet from outside invasion, they created a twenty-nine points resolution called “Imperial decree of protective procedures for Tibet”. The fourth clause of this decree stated “The lack of official military in the region of Tibet has lead to emergency drafts in time of crisis, which has proven to be harmful to the Tibetan people. The emperor has approved for Tibet to form a official troop of three thousand men. One thousand each will be stationed in front and back Tibet, five hundred in Jiangzi and five hundred in Dingri.” These three thousand troops became what is commonly known as the Tibetan Infantry. Considering that a military flag is a necessity for the daily training of this army, the central Qing government approved the “snow lion flag” as official military flag of Tibet.
This tradition is continued down to present day, where the Tibetan government in exile still uses the “snow lion flag” as their official representation.
中文维基
1912年，该旗首度启用。1918年，第十三世达赖喇嘛颁布，正式由拉萨的清政府采用。1950年，中华人民共和国政府与西藏政府签订《和平解放协定》，人民解放军进驻西藏后，该旗就由五星红旗取代。1959年，发生拉萨冲突，中国人民解放军占领西藏全境并下令取缔西藏政府，第十四世达赖喇嘛逃离西藏后，该旗被中华人民共和国视为西藏独立运动的标识，被严格禁止展示或使用。
９世纪中叶，吐蕃王朝分裂后，在很长一段时期里，西藏没有出现正规的军队，也就没有正规的军旗了。直到１８世纪末叶，清朝在派兵击退外敌对西藏的入侵后，制定了著名的《钦定藏内善后章程》二十九条。其中第四条规定：“以前前后藏都没有正规军队，用时临时征调，不仅缺乏作战能力，并且造扰人民，为害很大。
这次呈请大皇帝批准，成立三千名正规军队：前后藏各驻一千名，江孜驻五百名，定日驻五百名。这支钦定额为三千的军队，就是后来人们常说的“藏军”。军队平时的训练和作战的指挥，都离不了军旗。于是”中央政府清朝政府制作了”藏军的军旗“雪山狮子旗”。这就是藏独所谓“国旗”和“军队”的来历。
目前，位于印度达兰萨拉的西藏流亡政府继续使用雪山狮子旗。