A hydrogen sensor includes a substrate, a hydrogen absorbing member, a power source, an ammeter, and a processing unit. The hydrogen absorbing member is positioned on the substrate, and includes a carbon nanotube yarn and a hydrogen absorbing film coated on the carbon nanotube yarn. The ammeter measures an electric current. The hydrogen absorbing member and the ammeter are connected in series to the power source. The processing unit is electrically coupled to the ammeter to obtain a hydrogen concentration according to the electric current.