The precision of gas detection is affected by each function module performance of the system, when tunable diode laser absorption spectroscopy (TDLAS) is applied in gas detection. So the output current noise spectral density and the responsivity characteristics of photo detector are studied. The paper deduces the expression of the output current of photo detector, and the conclusion that the current is related to laser relative intensity noise (RIN), which exits in TDLAS experimental system. Effect of RIN on the output current of photo detector is studied by simulation, and the curve of current noise spectral density on different conditions is given. To avoid the photo detector responsivity being affected by ambient temperature, a real- time correction method is designed with the given principle and formula. Taking ammonia gas as an example, the concentration curve is corrected by using the method.