Abstract: Well loss is usually seen in drilling operation.It delays the safe drilling process and costs too much.With the analysis of well loss,the simulation apparatus is built up and heat generating and controlling equipment is designed.A set of data acquisition and processing system is developed as well.With all the equipment,the thermal characteristics of the flow during well loss are investigated.After the experiment,a new device of detecting the well leaking interval is developed.It can be used without tripping out the drilling stem and can heat the drilling stem with the control signal on the land.It can measure the temperature data and compare them before heating and after heating,determining the flow mode in the annular through the deference of temperature data.The well leaking interval can be found by changing the depth of the device.The new approach is accurate and the new device is small,which can provide a technical support for safe drilling.