Abstract: According to the present disclosure, a measuring method of liquid mixture purity includes steps as follows. A storage tank is provided, wherein the storage tank is configured for storing a liquid mixture including formic acid and water. A calculating unit is provided, wherein a plurality of formic acid purity values are saved in the calculating unit. A pressure-decreasing and heating step is performed by reducing a pressure of the storage tank and heating the storage tank. A measuring step is performed by measuring in the inner space of the storage tank to obtain a pressure value, and measuring the liquid mixture simultaneously to obtain a temperature value. A calculating step is performed by inputting the pressure value and the temperature value into the calculating unit, wherein the calculating unit outputs one of the formic acid purity values corresponding thereto.