Precision displacement measurement is a critical element to achieve active control for high-precision (e.g. 5 μm) flexible line boring. Since current measurement methods are difficult to apply in this application, a new measurement method has been developed. This paper presents a new strain-gage based method for the high-precision vibration measurement of a beam. This method is based on the fact that the vibration displacement can be expressed in terms of an infinite number of vibration modes and related to the measured strains through the strain-displacement relationship. By placing multiple sets of strain gages, multiple modes can be taken into account to achieve high-precision measurement. Both simulation and experimental results were obtained and are provided for validation. From these results, it can be concluded that this new measurement method is applicable to high-precision flexible line boring, enabling active control to compensate for the tool-tip displacement to achieve a high-precision machining process.