Abstract: The real--time dynamic structure of Pd--D system in D_2O electrolysis cell is measured on neutronprowder diffractometer in CIAE. Diffraction patterns in 2θ range of 34°--95°are obtained under theconditions of electrolysing for 0,3 and 48 A·h respectively, and the gradual transition of Pd--D systemfrom α--phase to β--phase is observed. The real--time measurements of β peak of (220) reflection showthat intensity of β peak almost reaches the saturation point after electrolysing for 0. 65 A·h and increas-es slowly with further electrolysis afterwards.