Brillouin amplification of Stokes seed pulses with steep leading edge is investigated. Various parts of Stokes seed pulse can be selectively amplified by changing the delay time between pump and Stokes seed pulse entering the cell, and Stokes pulse shape can be modified. Dependences of output Stokes pulse duration, the temporal ratio of leading edge to trailing edge β, and the energy extraction efficiency η on the encounter time are theoretically and experimentally researched. Output Stokes pulse with β=1 and η>80% is theoretically achieved at encounter time of 2.5 ns. Theoretical simulations fit with the experimental results.