A tool for extracting liquid, comprising: a pressure variation apparatus, a liquid storage space, a liquid suction passage, and an outflow passage, wherein: the pressure variation apparatus comprises a pneumatic cylinder and a piston capable of reciprocating inside the pneumatic cylinder; the piston reciprocates to generate positive and negative pressure variations inside the pneumatic cylinder; a starting end of the liquid storage space is in communication with an interior of the pneumatic cylinder; the liquid suction passage is in communication with the liquid storage space, and a valve capable of being opened toward a direction of the liquid storage space and being closed toward an opposite direction is disposed inside the liquid suction passage; and the outflow passage is in communication with a tail end of the liquid storage space, and a valve capable of being opened toward a direction of an external space and being closed toward an opposite direction is disposed inside the outflow passage; characterized in that a temporary storage groove is arranged on a pneumatic cylinder wall whose horizontal position is lower than a port of the starting end of the liquid storage space; the temporary storage groove is a groove on an inner pneumatic cylinder wall, and when the temporary storage groove is located at the sealed part of the pneumatic cylinder, the temporary storage groove temporarily stores excess liquid that enters into the pneumatic cylinder; when the temporary storage groove is located outside the sealed part, liquid inside the temporary storage groove flows out under the action of gravity or as being swung by an operator; the position of the temporary storage groove makes a maximum volume of a sealed part of the pneumatic cylinder larger than a volume of the liquid storage space. the pressure variation apparatus comprises a pneumatic cylinder and a piston capable of reciprocating inside the pneumatic cylinder; the piston reciprocates to generate positive and negative pressure variations inside the pneumatic cylinder; a starting end of the liquid storage space is in communication with an interior of the pneumatic cylinder; the liquid suction passage is in communication with the liquid storage space, and a valve capable of being opened toward a direction of the liquid storage space and being closed toward an opposite direction is disposed inside the liquid suction passage; and the outflow passage is in communication with a tail end of the liquid storage space, and a valve capable of being opened toward a direction of an external space and being closed toward an opposite direction is disposed inside the outflow passage; characterized in that a temporary storage groove is arranged on a pneumatic cylinder wall whose horizontal position is lower than a port of the starting end of the liquid storage space; the temporary storage groove is a groove on an inner pneumatic cylinder wall, and when the temporary storage groove is located at the sealed part of the pneumatic cylinder, the temporary storage groove temporarily stores excess liquid that enters into the pneumatic cylinder; when the temporary storage groove is located outside the sealed part, liquid inside the temporary storage groove flows out under the action of gravity or as being swung by an operator; the position of the temporary storage groove makes a maximum volume of a sealed part of the pneumatic cylinder larger than a volume of the liquid storage space.