The episode is "The Love of My Life".

"The Love of My Life" is a song written by Tang Shuchen and composed and sung by Lu Guanting. It was composed as the ending theme of Stephen Chow's classic movie "The Marriage of the Monkey King". The song describes the growth of a boy into a man. The process expresses a deep-seated lovesickness that is both helpless and sad.

Extended information:

"The Love of a Lifetime" was co-created by Lu Guanting and his wife ***. When Lu Guanting was composing "The Love of a Lifetime", Liu Zhenwei said that this love was fruitless for five hundred years. Lu Guanting got goosebumps all over, and the melody came out when he got home at night. When Tang Shuchen was creating, he was also moved by "Five Hundred Years of Love and Suffering" and completed the creation in only two hours.

The theme song "The Love of My Life" composed by Lu Guanzhong gave life to this brand. Whenever the prelude sounds, it's like seeing Zixia saying "I can't guess the ending", then breaking away from the grasp of the Supreme Treasure and falling slowly. It was like seeing the yellow sand on the tower begin to roll up with the wind, floating to the feet of the lover who was embracing each other on the wall, and then blowing past the backs of the four masters and apprentices as they left for the west.