We report the experimental data of Hügoniot longitudinal sound velocity VL for natural(Mg.092,Fe0. 08)Si03enstatite sample at about 40-140 GPa,consisting of three new data and five previously reported data but revised by our new Hügoniot equation of state parameters.Three segments,separated by two discontinuities,appear in the VL-pH(shockpressure)plot.Analyses show that the first discontinuity at about 64 GPa,with a sharp increase of VL of about 21％,is judged to be a phase transition from enstatite to Pbnm perovskite (PV)；while the second one at about 83 GPa,with a dramatic decrease of VL of about 23％,is likely caused by a subtle structural change from Pbnm PV to tetragonal PV, accompanied by material strength softening due to melting of oxygen sublattices.This strength softening evidence is obtained first from shock wave experiments,and probably has profound implications for probing into the origin of low seismic velocity anomaly in the Earths lower mantle and thus constraining the geophysical and geochemical models for the Earths lower mantle.