Abstract: With the increasing urgency of preventing and controlling heavy metal pollution in soil sites, on-site, rapid, and in-situ screening technologies have emerged as critical components of environmental monitoring. Si-PIN detectors, distinguished by their low cost, compact size, and high stability, have become the core components of portable X-ray fluorescence (XRF) spectrometers used for on-site soil heavy metal detection. However, their key performance parameter—energy resolution—is highly susceptible to the synergistic effects of complex field ambient temperatures and signal peaking times. Existing research predominantly examines single factors in isolation, lacking a systematic understanding of the interaction mechanism between these variables. Consequently, parameter configuration in practical applications involving complex soil matrix analysis often relies on empirical experience rather than theoretical support. Furthermore, the performance of domestically produced detectors in analyzing complex samples such as soil, as well as the quantitative gap in core parameters compared to mainstream international products, remains unclear, hindering their high-end application and technological upgrading. To address these issues, this study focuses on the PA200 series Si-PIN detector components independently developed by Nucleus Photonics. It aims to reveal the synergistic laws of temperature and peaking time and quantitatively assess their actual performance in soil heavy metal detection. A controllable experimental platform was established, employing an orthogonal experimental design to systematically investigate the interactive effects of temperature and peaking time on energy resolution and peak position stability. National standard geological reference materials (GBW07105 and GBW07106) were selected to represent complex soil matrices for practical application testing, alongside parallel comparative experiments with the internationally mainstream Amptek Si-PIN detector. The results indicate that at a constant low temperature of 250 K, the relative measurement deviation for the characteristic peaks of Mn, Cu, and Ag is less than 0.01%, demonstrating excellent peak position consistency. A significant non-linear coupling pattern exists between temperature and peaking time; an optimal energy resolution of 180–200 eV (@Mn Kα) can be achieved by combining a low-temperature range of 235–255 K with a peaking time of 20–30 μs. Comparative experiments demonstrate that the PA200 detector exhibits qualitative screening performance for multi-elements (e.g., Fe, Cu, Zn, Pb, As) in complex geological samples comparable to international counterparts, although certain gaps remain in the count rate and separation of weak signals. This study elucidates the synergistic mechanism of “suppressing thermal noise via low temperature as the foundation, and utilizing optimized peaking time for fine filtering”. It identifies the key parameter ranges for optimizing the performance of domestic Si-PIN detectors, providing a theoretical basis and experimental support for their precise application and performance enhancement in portable XRF equipment for on-site soil heavy metal detection.