Abstract: Tank microcosm has been used to study the effect of rainwater on contamination process of small landscaping water. Under the condition that there is no other external pollution is added, the effect of nutrient releasing from sediment on contamination transition is inspected. Result shows that rainwater makes NH₄⁺ concentration raised up in short time and leads to the nitrification of NH₄⁺. The increase of organic contamination consumes dissolved oxygen in water and weakens the nitrification, that leads to accumulate the middle production NO₂^- and delays the transition to NO₃^- also. For phosphate in rainwater, which has low self cleaning capability, combination of PO₄^3- and Ca²⁺,Mg²⁺ makes it more stable and hard to transit. The participation of rainwater increases the negative and positive ion, prolongs the period of ion releasing at water-bottom interface, and makes great fluctuation of pH at cover water. Above factors prolong the time of system stabilization.