The instant disclosure provides a gas detection device and method for detecting gas concentration. The gas detection device includes a chamber module, a light emitting module, an optical sensing module, and a light splitting module. The chamber module includes a light guiding chamber, a first sampling chamber, and a second sampling chamber. The light emitting module is disposed in the light guiding chamber to generate a projection light beam. The optical sensing module includes a first optical sensing unit disposed in the first sampling chamber, and a second optical sensing unit disposed in the second sampling chamber. The light splitting module is disposed in the chamber module. The projection light beam is split by the light splitting module to generate a first split light beam and a second split light beam.