Abstract: To comprehensively improve the protection quality of black-soil farmland and to fully release the social and ecological benefits of black-soil farmland protection are the internal requirements for consolidating the grain production capacity and promoting the green development of agriculture in Northeastern China. As an important resource for farmers to embed into the social structure, social capital plays a vital role in the formation of farmers’ willingness to protect black-soil farmland. Based on a survey date of farmers in Suihua City of Heilongjiang Province, and applying the theory of social capital, this paper analyzed the level of farmers’ willingness to protect black-soil farmland by the double hurdle model and discussed the influencing mechanism of social capital on farmers’ willingness to participate in and willingness to pay for black-soil farmland protection. Results show that the level of most farmers’ willingness to pay is between 1 181.40 to 1 859.85 yuan/(hm2?a) based on their willingness to participate in black-soil farmland protection. Social network and social participation positively stimulated farmers’ willingness to participate in black-soil farmland protection. While strong relationship network, the intensity of social participation, and the scope of social participation significantly promoted farmers’ willingness to pay for black-soil farmland protection. In addition, the general trust in social trust positively affected farmers’ willingness to participate in black-soil farmland protection. Although special trust inhibited farmers’ willingness to participate in black-soil farmland protection, it positively stimulated farmers’ willingness to pay for black-soil farmland protection. Therefore, this paper suggests that the government should enhance the level of social capital of farmers, build a platform for information exchange within rural communities, and improve the black-soil farmland ecological compensation policy.