An initial positioning system and method for liquid measuring and taking. The system comprises a measuring passage, a variable pressure chamber, and a piston, an initial end of the measuring passage being in communication with the upper part of the variable pressure chamber; the piston reciprocates in the variable pressure chamber to cause a change between positive pressure and negative pressure in the variable pressure chamber; the initial positioning system further comprises a pressure relief port located on the wall of the variable pressure chamber and capable of communicating with the outside of the variable pressure chamber; when the piston is located at the pressure relief port, and the inside and outside of the variable pressure chamber are in communication, the maximum volume of the liquid that can be accommodated in the variable pressure chamber is not smaller than one third of the total volume of the variable pressure chamber.