The aim of this investigation was to study the effects of different ultrasonic powers and treatment times on particle size and surface topography of low ash fine lignite (<0.125 mm) combined with flotation, wet-screening, surface roughness, scanning electron microscopy (SEM), and induction time tests. Flotation results show the concentrate yield gradually decreased with the increase of ultrasonic power and treatment time except 18 W. The wet-screening results indicate that increasing ultrasonic power and treatment time had more significant crushing effect on lignite particles and produced more fine particles (<0.045 mm), which was harmful for the improvement flotation performance of lignite. SEM results show that crack was gradually expanded into a gap as well as some new cracks and cavities on the surface of lignite are produced with the increase of ultrasonic power. The generation of these gaps, new cracks, and cavities increases the roughness of lignite surface. The higher the ultrasonic power is, the rougher the lignite surface becomes, which promotes its surface to entrap more water during the flotation process resulting in a formation of thick hydration shell. The induction time increased as ultrasonic power and treatment time increased except 18 W. Hence, there will be a negative impact on lignite flotation if the lignite particles are treated by higher ultrasonic power for longer. The findings of this paper may provide some guidance to understand the effects of ultrasound on surface topography of lignite and makebetter use of ultrasound to assist coal flotation.