An in-line oscillating device is essentially composed of an oscillating tank, an oscillating pipe, and an oscillating generator. The oscillating pipe is set in the oscillating tank and connects with a slurry pipe. The oscillating generator for generating ultrasonic waves is mounted on the oscillating tank. Furthermore, the oscillating tank is filled with a medium to transmit the ultrasonic waves generated by the oscillating generator to the oscillating pipe. The in-line oscillating device is suitable to be mounted on any location of the slurry pipe where oscillation is needed.