The uncertainties of blade tip displacement measurement using BTT are investigated through experiments. A novel test bench is established where the blade deformation can be measured simultaneously by laser displacement sensors and BTT, which enables the determination of the BTT measurement error. It is discovered that the rotational speed fluctuation is the primary factor that degrades the accuracy of BTT. The magnitude of error is dependent on the probe position and could be up to 0.5 mm even when the rotor is driven by a servo. Fortunately, it is also validated that using multiple keyphasors could effectively elevate the accuracy of BTT measurement.