In this article, a novel liquid flow sensor which is composed of a special structure conduit main body and a differential pressure sensor was designed, fabricated, and calibrated. The conduit main body includes an inlet channel section with a branch conduit connecting one end of the pressure sensor, a throat channel section, and an outlet channel section with a branch conduit connecting another end of the pressure sensor. The basic principle is to employ a differential pressure sensor to measure the pressure difference between the inlet channel and outlet channel of the conduit main body when fluid passes through it. The pressure difference between the two ends of the differential pressure sensor (i.e., the two branch conduits located in the inlet and outlet channel sections) is of either forward or backward flow and directly interrelates with the volume flow rate (mass flow rate or flow velocity) via the conduit main body, so the volume flow rate or mass flow rate or flow velocity can be calculated and the flow direction can be determined from the detected pressure difference. This liquid flow sensor is characterized by using only one differential pressure sensor of a simple structure, the error of which is less than 1%.