China has made remarkable progress in poverty alleviation. However, the key to consolidating the existing poverty alleviation achievements is how to prevent rural households out of poverty from regressing back poverty; in other words, how they can maintain livelihood sustainability? According to a theoretical analysis of livelihood sustainability, based on the “whole process” of the livelihood development of rural households out of poverty in China, this article puts forward three determinants of livelihood sustainability – livelihood basis, livelihood acceleration and livelihood environment, establishes the dynamic mechanism framework from the perspective of the “acting force” and a corresponding evaluation indicator system. Taking Hua County of Henan Province, one of the first poverty alleviation counties in China, as an example, according to the questionnaire of rural households out of poverty, this paper analyzes the current static livelihood characteristics of rural households out of poverty and the dynamic mechanisms of livelihood sustainability in the process of poverty alleviation on the basis of the quantitative measurement of livelihood sustainability. The main conclusions are as follows: (i) Livelihood basis, livelihood acceleration, livelihood environment and livelihood sustainability of rural households out of poverty in Hua County are at the medium level as a whole. (ii) Livelihood resources, especially labor resources, and the initiative of rural households to develop their livelihood, have important influences on sustainability; rural households that lifted themselves out of poverty earlier, are located in characteristic industrial zones or modern industrial zones, or adopt diversified or non-agricultural production modes, have a higher level of livelihood sustainability. (iii) The sustainable livelihood development of rural households out of poverty needs to stimulate the endogenous power of rural households to generate positive livelihood acceleration on the premise of securing a livelihood basis, supplemented by suitable livelihood environment support. In the process of poverty alleviation, the livelihood basis, livelihood acceleration and livelihood environment play the roles of the “starting force”, “driving force” and “supporting force” respectively in order to jointly affect the livelihood behavior of rural households, form different levels of livelihood performance, and present different dynamic processes of livelihood sustainability. These findings are of great value for other developing countries' improving livelihood sustainability of rural households.