Owing to the lack of terrestrial heat flow data, studying lithospheric thermal structure and geodynamics of the Yingen-Ejinaqi Basin in Inner Mongolia is limited. In this paper, the terrestrial heat flow of the Chagan sag in the Yingen-Ejinaqi Basin were calculated by 193 system steady-state temperature measurements of 4 wells, and newly measuring 62 rock thermal conductivity and 20 heat production rate data on basis of the original 107 rock thermal conductivity and 70 heat production data. The results show that the average thermal conductivity and heat production rate are 2.11±0.28?W/(m·K) and 2.42±0.25?μW/m3 in the Lower Cretaceous of the Chagan sag. The average geothermal gradient from the Lower Suhongtu 2 Formation to the Suhongtu 1 Formation is 37.6 °C/km, and that of the Bayingebi 2 Formation is 27.4 °C/km. Meanwhile, the average terrestrial heat flow in the Chagan sag is 70.6?mW/m2. On the above results, it is clear that there is an obvious negative correlation between the thermal conductivity of the stratum and its geothermal gradient. Moreover, it reveals that there is a geothermal state between tectonically stable and active areas. This work may provide geothermal parameters for further research of lithospheric thermal structure and geodynamics in the Chagan sag.