Photo-bleaching of the silica-related bismuth active center (BAC-Si) in bismuth/erbium co-doped optical fibers is investigated. By analyzing dynamic spectral characteristics of BAC-Si, the photo-bleaching of the BAC-Si is found to be linked to the escape of an excited electron from the bismuth site in the BAC-Si. This mechanism of BAC-Si bleaching linked to an escaping excited electron is further confirmed with both photo-bleaching experiments by different laser pump wavelengths and a potential energy model describing the loss of an excited electron. Additionally, the temperature effect on the photo-bleaching, which is in good agreement with the above findings, is observed and discussed.