The invention provides a liquid filling device, which belongs to the technical field of operation and solves the problems of complex operation and poor performance existing in the traditional liquid filling device. The liquid filling device comprises a liquid storage box, a pneumatic diaphragm pump, a filling connector which can be communicated with a filling opening and an air inlet connector which can be communicated with an air supply opening, wherein a liquid inlet of the pneumatic diaphragm pump is communicated with the liquid storage box through a liquid inlet pipeline assembly and is positioned at the bottom part of the liquid storage box; a liquid outlet of the pneumatic diaphragm pump is communicated with the filling connector through a liquid outlet pipeline assembly; and the air inlet connector is communicated with an air inlet of the pneumatic diaphragm pump through an air inlet pipeline assembly which can be used for enabling the air to enter into the pneumatic diaphragm pump in voltage stabilization and controlling the air inlet or the air cut. The liquid filling device disclosed by the invention has the advantages of compact structure, convenience for control, high injection efficiency, and time and labor saving; and by adopting the pneumatic diaphragm pump as a power source, the liquid filling device has the advantages of safety in use, stable performance and the like.