A gas detecting apparatus and a gas detecting method based on terahertz spectroscopy are provided. The apparatus includes a sample chamber allowing a terahertz wave to pass therethrough; a gas feeding unit connected to the sample chamber to feed gas into the sample chamber; a gas outputting unit connected to the sample chamber to output gas from the sample chamber; and a vacuum pump connected to the sample chamber to evacuate the sample chamber. The apparatus further comprises one or more of a pressure gauge disposed on the sample chamber, an anemometer disposed on the sample chamber, a humidity regulation device connected to the sample chamber, and a temperature regulation device connected to the sample chamber.