With the self-developed devices for in-situ soil sample preservation and desorption in oil and gas geochemical exploration and through optimizing the detecting conditions by such means as using saturated salt water or pure water, changing the sample weight, desorbing time, and negative pressure conditions, the authors obtained the best condition of analyzing. According to the experimental results, the method for in-situ soil sample preservation and desorption in oil and gas geochemical exploration was established. Precision waas used to evaluate the method. The result shows that this method has high precision, as shown by its good performance in a known area.