Abstract: The concept of ‘lucid waters and lush mountains are gold and silver mountain of invaluable assets’ (known as the ‘Two Mountains’ transformation) proposed by Xi Jinping outlines a series of progressive development processes for China to achieve sustainable development. To evaluate the performance of regional ecological civilization construction, it is essential to establish a method for identifying the stage of the ‘Two Mountains’ transformation. In this study, a methodology was developed by using threshold identification and sliding t-test to determine the stages of the ‘Two Mountains’ transformation in three specific regions: Tangshan City, Nantong City, and Bama Yao Autonomous County. The research results show that: (1) The critical point of resource and environmental costs slow down with economic growth is the basis for identifying the transition from the stage of ‘lush mountains in exchange for gold mountains’ to ‘valuing both lush mountains gold mountains’. The peak inflection point further indicates the shift towards the stage of ‘preferring lush mountains over gold mountains’. (2) As the cost of the resources and the environment continues to decrease, the critical point where the intangible value of ecosystems becomes prominent becomes the core identifying the effectiveness of ‘lush mountains are gold mountains’. (3) Different regions have different paths to achieve the ‘Two Mountains’ transformation. The eastern region takes the lead in implementing the strategy of pollution first and treatment later, while the west region, which has a better environment but is economically backward, requires national ecological compensation and cooperation with advanced regions to achieve leapfrog development. The research shows that analyzing the evolution of the relationship between environment and economy is an important basis for identifying the regional ‘Two mountains’ transformation stages.