To study the influence of three common water plants decomposition on water quality in crab eco-culture pond, a 60-day interior simulation experiment was carried out in barrels. The results indicated that decomposition rates of three kinds of plants have the same characteristics, it was rapid in the early but slow in the middle and later periods. But they were different, the decomposition rate of Hydrilla verticillata and Elodea nuttallii were approximated and faster than Ceratophyllum demersum. At the end of experiment, the mass loss rate of Hydrilla verticillata, Elodea nuttalliiare and Ceratophyllum demersum were 72.3%±2.1%,71.7%±1.5% and 58.3%±0.6% . Water quality factors changed obviously in the early of decomposition and water was from neutral to acid. COD increased by about 4.5 times than start in the water of three kinds of water plants and the water was yellow and smelly. DO was greatly consumed, the water was in a hypoxic or anaerobic environment. Denitrification was promoted, so NO₃^--N decreased rapidly but NO₂^--N and NH₄⁺-N increased rapidly, which NH₄⁺-N was six times than initial concentration. TN and TP increased significantly and TP was the biggest change in all the water factors. The TP increased about 123,124 and 66 times in the water of Hydrilla verticillata、Elodea nuttallii and Ceratophyllum demersum on the third day. Along with the experiment, part of nitrogen and phosphorus in the water deposited into the sediment. So, the large biomass residues of submerged plants should be controlled avoid secondary pollution caused by excessive decomposition of submerged plants.