When fluid flows through a capillary，head losses will be occurred，which relates to the length of the capillary. With this principle，we make the liquid flow through two capillaries，whose inner diameters are the same while lengths are different. Controlling the same perpendicular distances from the outflow end of capillaries to liquid level，and measuring the ratio of liquid mass flowing from two capillaries in the same time，the viscosity of this liquid can be obtained theoretically by calculation.