Most gas molecules have the fundamental vibration absorb spectrum in the mid-infrared region during 3~5 μm.Detecting the gas concentration using the mid-infrared semiconductor opto-electronic devices have more advantages,such as high sensitivity,high accuracy and stability etc.The optical software TracePro is a simulation and analysis tool that performs stray light analysis,illumination analysis,and optical systems analysis.This paper designs a dual path structure using TracePro for mid-infrared gas sensing system.Using TracePro to simulate and analyze this optical design and modify the design according the analysis report.