A　series　of　a-Si:H　films　are　deposited　by　hot　wire　assisted　microwave　electron　cyclotron　resonant　chemical　vapour　deposition　(HW-MWECR-CVD),　subsequently　exposed　under　simulated　illumination　for　three　hours.　This　paper　studies　transformation　tendencies　of　microstructure　after　illumination.　It　proposes　a　model　of　light　induced　structural　change　based　on　the　experimental　results.　It　is　found　that　all　samples　follow　the　same　mechanism　during　illumination,　and　intrinsic　structure　of　samples　affect　the　total　H　content.