With the extension of urbanization to county areas in China, urban renewal in southern county-level cities has become a measure to promote the integrated development of urban and rural areas. As the core of urban renewal, the renovation of old communities faces prominent problems such as frequent leakage and large wall settlement caused by outdated renovation technologies, which seriously affects the quality of life of residents and the improvement of urban functions. Focusing on the renovation of old communities in the context of urban renewal at the county level in southern China, this paper conducts targeted research on key technologies such as improving the quality and efficiency of sewage systems, repairing exterior walls, and renovating waterproofing roofs. Through formulating scientific renovation plans and implementing engineering practices, the renovation effect is verified with actual measurement data. The research results show that after the renovation, the core indicators such as the thickness of the roof waterproof layer and the amount of wall settlement are better than the standard specifications, which proves the scientificity and effectiveness of the proposed plan and provides technical reference and practical paradigm for the renovation of old communities in southern county-level cities.