A gas concentration detecting device and a detecting method thereof are provided. The gas concentration detecting device includes: a first polarizer; a second polarizer arranged opposite to the first polarizer in such a way that a gap is formed between the first polarizer and the second polarizer; a light source on a side of the second polarizer away from the first polarizer or on a side of the first polarizer away from the second polarizer; and a photoluminescent layer between the first polarizer and the second polarizer. The light transmission axis of the first polarizer is perpendicular to a light transmission axis of the second polarizer.