The present invention discloses a respiration sensor, which is attached to a testee and comprises a triaxial acceleration sensing unit, a microprocessor, and at least one alarm unit. The triaxial acceleration sensing unit detects the respiration-related movements of a testee and output a detection signal. The microprocessor connects with the triaxial acceleration sensing unit, receives the detection signal and transforms the detection signal into a respiration signal to determine the respiration state of the testee. When the respiration signal is lower than a standard, the microprocessor triggers the alarm unit to give out an alarm to alert the persons nearby.