In response to the problem of cost and benefit imbalance in the construction of low impact development (LID) rainwater systems, a residential area in Tianshui City, Gansu Province was selected as the research area to construct an InfoWorks ICM one and two dimensional coupled model. The total runoff control rate, peak runoff reduction rate, water accumulation area reduction rate, water accumulation reduction rate, suspended solids (SS) reduction rate and chemical oxygen demand (COD) reduction rate of different LID combination schemes under different rainfall return periods were quantitatively analyzed for the study of cost-effectiveness of LID facilities, with the consideration of their life cycle cost. The results show that the LID combination scheme with green roof is less cost-effective, but it has a strong adaptability to heavy rainfall, and is more suitable for rainstorm prone areas. The LID combination schemes are highly cost-effective in reducing the area and volume of water accumulation, and can effectively alleviate waterlogging; however, the cost-effectiveness of COD reduction rate is relatively low, and further optimization is needed. The combination of rainwater buckets, biological retention facilities and permeable paving, as the most cost-effective solution, is more suitable for the construction of LID rainwater systems in the study area. The research results have important reference significance for the optimization design of LID rainwater systems.