It is of great significance to explore the impacts and mechanism of social capital on the investment efficiency of farms to increase their income from grain production and to enhance the comprehensive production capacity. Based on a survey data of 401 households in six counties (districts) of Jiangsu Province in 2021, this paper analyzed the mechanism and the impacts of social capital on the investment efficiency for rice farms. Results show that the investment efficiency of the sample farms is low. The average output and income efficiency are 0.66 and 0.50, respectively. All different types of social capital have significant positive effects on the investment efficiency. Kinship and identity social capital improve the investment efficiency by promoting the productive credit, while organizational and entrepreneurial social capital play a role by expanding sales channels. Heterogeneity analysis shows that there are significant differences in the impacts of social capital on rice farms of different investment efficiency and in different labor mobility villages. Therefore, this paper suggests that the government should promote farms’ investment efficiency steadily by cultivating new agricultural management entities, implementing the multi-linked loan model, and accelerating the construction of the agricultural whole industry chain.