The invention discloses an online heparin sodium viscosity detecting device and method. The device comprises a liquid collecting pipe, wherein a rotor is fixed in the liquid collecting pipe through a bracket and connected with the bracket through a rotating shaft, a rotor revolution number counting device is arranged on the bracket, a pressure pump is communicated with the liquid collecting pipe, and a liquid dilution bin is communicated with the liquid collecting pipe through an electromagnetic valve; the rotor revolution number counting device is connected with a controller. The method comprises liquid taking, pressurized pushing and distance measurement. The online heparin sodium viscosity detecting device and method have the advantages that the viscosity of a heparin sodium solution can be detected on line in real time, and the data detection accuracy is high.