Pré-publication, Document de travail: Using a large census dataset spanning 40 years, this paper presents the first comprehensive study of intergenerational absolute income mobility in Hong Kong, by employing the copula and marginals approximation method. The main findings indicate a significant decrease in absolute income mobility, declining from 85% in the 1976 cohort to 55% in the 1996 cohort. In 1976, Hong Kong's absolute income mobility(AIM) exceeded that of major higher-income countries, but within 40 years, it converged to the level of the United States and Europe. This decline is primarily attributed to decelerating GDP growth rather than increased income inequality. Our findings remain robust under various alternatives of copula forms and different birth cohorts. Notably, our innovative decomposition of the education factor reveals that education plays a crucial role in mitigating the decline in absolute mobility. We argue that the rapid economic growth of China and the expansion of Hong Kong's tertiary education play important roles in shaping intergenerational income mobility in Hong Kong.