Abstract: The thermal conductivity is the key parameter to the design of many projects, such as energy structures, high-voltage buried power cables and permafrost embankment, related to estimating the heat transfer capability and temperature field in the soil. However, at present there is no effective in-situ testing method. Based on the theory of instantaneous heat release along a line source, a heat conduction cone penetration test (CPT) probe for thermal conductivity evaluation of in-situ soil is developed. According to the theoretical assumptions and the sizes of CPT system, the length, diameter, internal structure and positions of the temperature sensors are introduced. Then, the corresponding test procedure and the method for thermal conductivity are proposed. The test process is simulated in COMSOL to verify the method, and the results validate that the actual heat transfer conforms to the line source theory. The interpretation method yields reasonable values within a general range of conductivities. For less conductive soil (<0.6 W/ (m·K)), longer duration of heat dissipation may be required. The field test results show that the in-situ soil conductivity is higher than that from laboratory tests on undisturbed samples, indicating the sampling disturbance may be responsible for this reduction. Finally, some suggestions on laboratory thermal conductivity tests and engineering designs are given.