The invention provides a device for separating rough and fine coal powder in a pipeline, comprising an air pipe, wherein a spindle is arranged at the part close to an entrance, of an inner axis of the air pipe; a through hole is arranged at the center of the spindle and an oil cavity pipe is inserted into the through hole; a loudspeaker-shaped separation channel is arranged at the part close to an exit, of the of the air pipe; a large-diameter end of the separation channel is arranged towards the spindle; and a gap is reserved between the separation channel and the side wall of the air pipe. When a coal powder air flow passes through the spindle, the coal powder with larger grains is guided to a pipe wall after passing through the spindle due to that mass inertias of the rough and fine coal powder grains are different and flowing tracks of the rough and fine coal powder grains along the air flow are different; the rough and fine coal powder can be separated through the separation channel in the device, so that the fine coal powder is concentrated at the center of the pipe and the rough powder is close to the pipe wall. After the rough coal powder and fine coal powder are separated through the separation channel, the fine coal powder is ignited by flame of an oil gun and then the rough coal powder is ignited by the heat released through the fine coal powder, thereby realizing tiny-oil ignition, gradual amplification and direct ignition of the coal powder and realizing the aim of obviously saving oil in an ignition process of a boiler.