The technology of non-invasive near-infrared spectral detection of biological tissue blood oxygen saturation has great research and application prospects, and has widespread clinical applications. However, the conventional finger oximeter only reflects local oxygen saturation values, and has limitations in the application. These oxygenation readings become unreliable or cease, too. In the present paper, a new method is described that the reflectance pulse oximetry with variable incident light intensity is employed to measure biological multilayer tissue. The photoplethysmographic (PPG) signals significantly change with the change in the incident light intensity in the experiment on the fingertip. Further analysis shows that the changes of the PPG signals correspond to the multilayer structure of finger. It means that the blood oxygen saturation of different tissue level must be calculated from the PPG signals. These features show that this method must be feasible.