A device for measuring blood oxygen content comprises a blood oxygen sensor interface, a signal processing unit, and a plurality of voltage sampling points. A method for detecting fault in a blood oxygen sensor comprises the steps of: connecting the blood oxygen sensor to the signal processing unit; providing voltage signals by the signal processing unit via a pair of testing terminals, for alternately driving two LEDs connected in inverse-parallel with each other in the blood oxygen sensor; sampling the voltages of the pair of testing terminals, respectively, and determining the fault relevant to the LEDs based on the relationship of the voltages; receiving current signals from a photodiode in the blood oxygen sensor by the signal processing unit via another pair of terminals, converting the received current signals into voltage signals, and outputting positive voltage signals via a reverser and a follower, respectively; and sampling the positive voltage signals respectively for determining the fault relevant to the photodiode. Thus, the fault of the sensor can be detected and determined by employing a plurality of sampling points without the tissue to be measured of the human body, thereby the output error due to the faults of the sensor can be avoided, and the device can be maintained conveniently.