Multithermal fluid (MTF) component ratios and injection parameters are critical inputs in offshore heavy oil development, such as injection adjustment and monitoring, productivity prediction, and generator combustion process optimization. We implement simultaneous in situ diagnostics of two emblematic injection parameters, the gas-water ratio (GWR) and noncondensable gases proportion (NCGP), in a pilot-scale environment. A system-level integration of a novel laser absorption spectroscopy multigas sensor system based on integrating stray radiation suppression and a circular cell-enhanced strategy is proposed. A structurally optimized extinction thread in front of a photodetector is designed to reduce the absorption signal distortion under the influence of high-temperature radiation. Meanwhile, we break the limitation of the internal dimensions of the injection tube on the long-path absorption of gas molecules and improve the absorbance signal SNR by 3.42-fold. The present work performed experimental tests using diesel as the primary fuel in a laboratory-scale MTF generation system. The results show that the measurement uncertainties for H