Speaking on Beijing Radio station, musical director Chen Qigang said the organisers needed a girl with both a good image and a good voice.
A television image of Yang Peiyi, who sang the song for the Beijing Olympic Games opening ceremony
Yang Peiyi had the perfect voice but apparently not the perfect face
They faced a dilemma because although Lin was prettier, seven-year-old Yang had the better voice, Mr Chen said.
"After several tests, we decided to put Lin Miaoke on the live picture, while using Yang Peiyi's voice," he told the radio station.
"The reason for this is that we must put our country's interest first," he added.
"The girl appearing on the picture must be flawless in terms of her facial expression and the great feeling she can give to people."
Singer Lin, who is being called the "smiling angel", has already become a media celebrity because of her performance.
She told state-run China Daily that she felt "beautiful" in the red dress she wore during the performance.
Her dad told the newspaper that she already had fans all over the country.
According to Chinese news reports, Yang said she did not regret the decision, saying she was satisfied to have had her voice featured in the opening ceremony.
This is the second "fake" story about the opening ceremony
Viewers around the world saw a display in which 29 firework "footprints" travelled across Beijing from south to north.
But a senior official from the Beijing organising committee (Bocog) confirmed on Tuesday that footage of the display had been produced before the big night.
This was provided to broadcasters for "convenience and theatrical effects", according to Wang Wei, Bocog's executive vice-president.
"Because of poor visibility, some previously recorded footage may have been used," he told a daily press conference.