It is not known when Song Guangsi was born or how he became an eunuch, but it is known that he was from Fu Prefecture (福州, in modern Fuzhou, Fujian).
The first reference to him was that he was serving under Wang Jian's daughter, whom Wang created the Princess Puning (according to the Spring and Autumn Annals of the Ten Kingdoms (十國春秋) or Princess Puci (according to the Zizhi Tongjian) after Former Shu was founded,[4] but whom he had married, in 905, before the founding in the state, to Li Jichong (李繼崇) the military governor (Jiedushi) of Tianxiong Circuit (天雄, headquartered in modern Tianshui, Gansu), who was a nephew of Wang's ally Li Maozhen the Prince of Qi.
However, after she arrived in Chengdu, Wang kept her there and did not return her to Li Jichong, and he kept Song under his service in the palace.
That year, when Wang fell ill, another eunuch, Tang Wenyi (唐文扆), tried to seize control of the palace and take over the governance of the state.
Wang Zongbi was described to be corrupt, and Song described to be engaging in frequent flattery; the two were thus blamed by traditional historians for the decline of the Former Shu state.
Wang Yan, however, did not initially consider this attack to be a major threat even after Wuxing Circuit (武興, headquartered in modern Baoji, Shaanxi) had fallen, and Wang Zongbi and Song further assuaged him by stating that all he needed to do was to station the main Former Shu army at Li Prefecture (利州, in modern Guangyuan, Sichuan) to block the path of the Later Tang forces, Later Tang would not dare to advance deeply into Former Shu territory, as Former Shu still maintained sizable garrisons at Shannan (山南, headquartered in modern Hanzhong, Sichuan) and Dongchuan (東川, headquartered in modern Mianyang, Sichuan) Circuits.