The coaxial measurement of electrical conductivity was improved by using shock initiation method,and the conductivity formula was derived. The suitable shock wave pressure was input to limit the shock wave reverberation in reaction zone and instability in detonation growth. And the conductivity curves of cast TNT and TNT/RDX compositions were studied. The chemical reaction times and the reaction zone widths were acquired by analyzing the inflexions in curves. Results show that the maximal conductivity decreases with the increasing of RDX in cast TNT/RDX compositions. The reaction time of cast TNT is about 0.08 μs, and the reaction zone widths of cast TNT and several TNT/RDX compositions are about 0.4 mm and 0.5 mm respectively.