Cambodia is at an early stage of development, with 21% of people presently living in cities. Phnom Penh, the capital and largest city of Cambodia, is under urbanization pressure with a population of 2.1 million and the annual growth rate of 3.2% in 2019. In this regard, aging infrastructure needs an upgrade or replacement with a new design considering a percentage, as high as possible, of permeable surfaces in urban areas. Low Impact Development (LID), including green infrastructure, should be taken into account in planning and design approaches to mitigate land development impacts on the environment. This study aims to evaluate the efficiency of LID scenarios on surface runoff reduction, peak flow reduction, and pollutant removal under rainfall patterns using PCSWMM model in Boeng Trabek sewerage system, Phnom Penh. Flow monitoring and water quality sampling during three rainfall events were conducted in a main conduit for testing model performance. Six types of LIDs (Infiltration trenches, bioretention, porous pavements, rain garden, green roof, and rain barrels) were implemented in an applicable proportion of existing sub-catchments. For every selected rainfall event, LIDs could reduce in average 48% of surface runoff, 35% of peak flow and increase infiltration rate to 90%. For water quality (COD, NO3, PO4, and TSS), the average sub-catchment’s washoff removal and outlet’s total pollutant removal is 55%. In summary, the implementation of LIDs has a significant impact on runoff reduction, peak flow reduction, and pollutant removal. The results provide concrete evidence for relevant stakeholders to consider Low Impact Development technique for sustainable development and to achieve smart cities.