Disclosed is an online sliver separating device for high moisture content cut tobaccos. The online sliver separating device is mainly composed of a feeding vibration tank, a cut tobacco opening device, a high frequency vibration tank, a two-stage air separator and a discharging vibration tank which are sequentially arranged along the conveying direction of the cut tobaccos. Two opening devices are arranged in a first air separating channel and a second air separating channel which are communicated with each other in the two-stage air separator. Upper pin rollers and lower pin rollers are staggered and are specifically arranged on the opening devices to drop the cut tobaccos to achieve the loosening of the cut tobaccos. A separating method of the online sliver separating device includes that the high moisture content cut tobaccos which are cut in tobacco cutting production lines pass through the cut tobacco opening devices and then are subjected to high frequency vibration by means of the high frequency vibration tank to enable the cut tobaccos to be in wide cut, thin spread and fluffy states, then the cut tobaccos enter into the opening devices in the two-stage air separator, and finally slivers in the cut tobaccos are separated out. By means of the device and the method for separating the silvers, less broken cut tobaccos are produced in the separating process, and the ratio for rejecting the slivers out of the cut tobaccos is obviously increased.