A liquid amount control device includes a position detector, a pump, a driver and a controller. The pump is connected with a first tip and configured to suck up the liquid out of the test tube. The driver is connected with the pump and configured to drive the pump to move. The controller is configured to: (1) control the driver to drive a first suction end of the first tip to enter a detection region of the position detector to obtain an end height position of the first suction end; (2) control the driver to drive the first suction end of the first tip to enter the liquid by a first entering depth according to the end height position; and (3) control the pump to suck up the liquid in the test tube through the first tip.