The voltage tunability of Pb(Fe1/2Nb1/2)1-xTixO3 single crystals is investigated at a low electric field (<130 V/cm) in a low frequency range (< 1 MHz).The results show that the capacitance is strongly suppressed by the applied dc biases for both the rhombohedral sample and the tetragonal sample.A negative voltage tunability is only detected in the tetragonal sample.The origin of the giant tunability and the negative tunability is discussed based on the multipolarization-mechanism model and the equivalent circuit model, respectively.It is ascribed to the interracial polarization at the interface of electrode/sample.