The ten-year fishing ban on the Yangtze River is an important measure to implement the concept of “ecological priority and green development”. As the ultimate implementer and the most important beneficiary of the fishing ban policy, the income security of fishermen after withdrawing from fishing is directly related to the smooth promotion of the policy. Based on the sustainable livelihood theory and a field research data in 11 counties and cities in Hunan Province, this paper analyzed the impacts of livelihood capital on household income quality and its mechanism of action and explored the heterogeneity of the impacts among fishermen by the OLS model and the mediating effect model. Results show that in terms of the original accumulation of fishermen’s livelihood capital, social capital, human capital, physical capital, and financial capital were at a better level with averages of 1.57, 1.54, 1.51 and 1.50, respectively. While the level of natural capital was relatively low with an average of only 1.06. The income of fishermen dropped significantly after the withdrawal, with the average annual income falling from 103 100 yuan before the withdrawal to 64 300 yuan. Moreover, the income gap between fishing households increased, with the ratio between the highest and lowest quartile of the average annual income of fishermen rising to 8 from 4 before the withdrawal. Human, physical, financial, and social capital all had significant positive impacts on income quality with human capital having the greatest impact. Although financial capital positively determined the income quality, it inhibited fishermen’s employment efforts to a certain extent. In addition, participation in public affairs and land production materials had the strongest contribution to income adequacy and income structure, respectively. Government subsidy support had the greatest positive effect on income sustainability and knowledge. Ease of access to information resources had the most significant negative effect on the cost of income. For fishermen at different ages, the impacts of livelihood capital on income quality were heterogeneous. Therefore, this paper suggests implementing various policies to benefit farmers, putting fishing ban funds into priority areas, increasing the support of village “competent persons”, and providing assistance based on the age structure of fishermen.