Electric explosion of metal wire is an effective way to obtain the nanometer-sized metal powder particles, the deductive procedure directly affects the size of the metal powder particles. Nanosecond pulsed laser was used to observe the transient state of the explosion process. And based on the laser interference fringes, the image of the explosive area with clear edge was obtained according to the interference of the fringes in the process of electric explosion. The three-dimensional distribution of the metal powder concentration at different moment was calculated according to the transmittance of laser passing through the explosive cloud. Measuring results show that, the diameter of metal wires expands from 0.3 mm to 4.7 mm by 0.5 μs after energization, and expands to 28 mm by 18 μs. The maximum concentration of particles decreases from 3×1021 /cm3 to 1.1×1020 /cm3. Throughout the expansion process, the particle concentration distribution presents the multiple ring belts along the radial direction.