Enhancing the livelihood sustainability of out-of-poverty farming households in poor mountainous areas is not only the realistic demand for rural poverty alleviation in the new era, but also the urgent demand of rural revitalization. Taking into consideration livelihood capital, livelihood strategy and livelihood environment, this study established a livelihood sustainability evaluation index system of out-of-poverty farming households, and used the household survey data of the out-of-poverty households in the Longnan mountainous area to evaluate their livelihood sustainability and identify livelihood barriers. The results show that: 1) From river valley, the middle mountain to the high mountain area, and with the passage of time since a household was out of poverty, livelihood sustainability of the out-of-poverty households decreased in turn, and the livelihood sustainability of work-oriented type and agriculture-industry complementary type are higher than other farmers. 2) The proportion of farmers whose livelihood is unsustainable in Longnan mountain area is 28.83%, and the proportion of farming households with unsustainable livelihoods is higher in high-mountain area, traditional farming type, and late out-of-poverty households, while the proportion of farmers who have unsustainable livelihoods is lower in river valleys, agriculture-industry complementary type, work-oriented type, and early out-of-poverty households. 3) Out-of-poverty households with unsustainable livelihoods are all faced with multiple livelihood barriers, and nearly two-thirds of them are faced with multidimensional capital environment barriers and multi-factor barriers. 4) In view of the multiple livelihood barriers faced by the out-of-poverty households with unsustainable livelihoods, targeted multidimensional livelihood intervention should be implemented for different categories of households.