A physiological function detecting earphone includes an earphone body, an earplug mounted at one side of the earphone body and defining a window, a light processing module including a light sensing module towards the window, and a signal processing module. A detecting method using the earphone by a participant includes the steps of: the participant inserts the earplug in the ear canal thereof; the light sensing module senses the changes of light emitted through the window and then reflected by the wall of the ear canal; the light processing module processes the reflected light changes to get photoplethysmography (PPG) signals; and the signal processing module receives and processes the PPG signals to get physiological information of the participant.