In today's Qinghai plateau, there is a mountain called Riyue Mountain. There is a beautiful legend about the origin of this mountain. It is said that Princess Wencheng would leave Chang'an and marry Tubo far away. Emperor Taizong of the Tang Dynasty made a golden mirror for Princess Wencheng, the Sun-Moon mirror. The mirror is said to shine on the rivers and mountains of the Central Plains for the princess's homesickness. When Princess Wencheng's team came to the Tang-Fan junction, she looked west, but saw the grassland empty, no trace of people; took out the treasure mirror to the east, Chang'an City was full of flowers, but tears fell unconsciously. But thinking of Tang Dynasty and the overall situation of the friendship between Tibet and Tibet, she stepped into the border of Tubo and threw away the mirror of sun and moon. As the precious mirror "clattering" landed, there were two hills rising slowly. Later generations call these two mountains "Sun Moon Mountain".