The invention discloses a fiber expanding device and a fiber expanding method thereof. The fiber expanding device comprises a rack, a filament separating and yarn guiding mechanism is arranged at one end of the rack, a tightening transmission wheel is arranged on the portion, behind the filament separating and yarn guiding mechanism, of the rack, two tension roller sets are sequentially arranged on the portion, behind the tightening transmission wheel, of the rack, and a vibrating roller set is arranged between the tension roller sets; the vibrating roller set comprises vibrating rollers and vibrating devices arranged on the vibrating rollers and used for controlling vibration of the vibrating rollers, each vibrating roller comprises a main rotating roller which is located on the core part and is capable of rotating relative to the rack, and multiple fiber expanding rollers are connected to the outer surface of the main rotating roller in an annular matrix array mode; each tension roller set comprises a fixed roller installed fixedly and a movable roller capable of rotating relative to the rack, wherein a tension detecting device is connected to the fixed roller, heating devices are connected to the fiber expanding rollers, the fixed rollers and the movable rollers, and the surfaces of the fiber expanding rollers, the fixed rollers and the movable rollers are coarse. The fiber expanding device is capable of expanding fiber to be wide, thin and flat, simple in structure and convenient to maintain and saves space.