We present a tunable plasmon-induced transparency (PIT) metamaterial for manipulating the group velocity of terahertz (THz) waves. The metamaterial is composed of metal split rings and photoconductive silicon strips. The strong PIT effect with slowing down THz waves is generated by the bright-bright mode coupling between the high-order plasmon mode and the lattice surface mode via electromagnetic destructive interference. By varying the conductivity of silicon strips, the group slowing performance is dynamically tunable. The group delay can achieve beyond 20 ps with the group index as high as 592, showing the promising application for THz signal manipulation.