This technology is an external heating multi-tube rotary low-temperature pyrolysis process aimed at realizing the classification and efficient utilization of low-rank coal, which can simultaneously produce high-quality semi-coke, gas and tar. After the low-rank coal is added into the rotary pyrolysis kiln, it is evenly distributed to each heat exchange tube, the coal goes through the tube side, and the flue gas goes through the shell side. The main engine adopts a multi-tube structure, which not only effectively increases the heat exchange area, but also improves the heat exchange intensity and processing capacity of the equipment, and the material in the tube is heated evenly, which is easy to realize the control of the pyrolysis process.