Abstract: Thermal probes, a device to exchange heat with air and liquor convert, have characteristics of no engine loss and highly decreasing earth temperature around the thermal probes. The thermal probes have obvious effects for decreasing earth temperature under the foundation, increasing cold storage of earth foundation and protecting permafrost. It is a simple and suitable engineering measure for protecting permafrost. Now it has been widely used in Qinghai-Tibet Railway in permafrost area. In view of application of the thermal probes in the civil engineering, many researches in China and foreign countries are carried out. Many key technologies of thermal probes are solved, for example, making crafts, trail of model in low temperature, design and calculation and so on. But there are not any father researches on the relation among diameter of thermal probes, effect of decreasing earth temperature and producing cold quantity. In order to solve this kind of problems, a series of experiments in permafrost region along the Qinghai-Tibet Railway are performed. The test content includes three thermal probes with the same length but different diameters in Anduo experimental section. In given conditions, within certain range of diameter of thermal probes, the results show: (1) its producing cold quantity doesn’t increase with the increase of the diameter; (2) there seems to be the same decreasing earth temperature and producing cold quantity with different diameters; (3) after comprehensive consideration, the optimal diameter of thermal probes is 89 mm.