Two Mountains Theory refers to the concept of "Lucid waters and lush mountains are invaluable assets". As the top-level design of ecological construction in China, this theory has been practiced for more than ten years. The importance of this theory to ecological sustainable development has been widely recognized, but the research on the ecological construction effect and its influence on soil and water conservation effect is still insufficient. By introducing the balanced scorecard, this paper constructs the evaluation index system of the construction effect of "Two Mountains Theory", and effectively evaluates the green development effect of Kaihua County in the source area of Qiantang River from 2005 to 2020. The relationship between the practice effect of "Two Mountains Theory" and the development of soil and water control effect is further discussed by redundancy analysis. The research shows that during the 16 years of practicing the "Two Mountains Theory", the ecological construction effect of each dimension (including economic dimension, social dimension, construction dimension and learning dimension) has shown an obvious upward trend, but the fluctuation trend and amplitude are different. The comprehensive effect of Kaihua County from 2005 to 2020 can be divided into four stages. That is to say, it stayed in a poor stage in 2005—2007, entered a qualified stage in 2008—2011, entered a good stage in 2012—2016, and entered an excellent stage in 2017—2020. In addition, the construction effects of each dimension and the comprehensive dimension are positively correlated with micro-soil erosion to a certain extent, but negatively correlated with moderate, severe and extreme soil erosion, indicating that ecological construction has effectively promoted the development of soil and water control in the study area in the process of practicing the "two mountains theory". The results of this study provide theoretical guidance for the government to make precise policies to promote the mutual transformation between green green hills and Jinshan Yinshan, especially in the source areas of rivers with serious soil erosion.