Agricultural planting structure not only affects the quality of agricultural supply system, but also is the focus of agricultural supply side structural reform. Based on the panel data of farmers in nine provinces of National Fixed-site Rural Survey from 2004 to 2015, this paper explores the impact of off-farm employment on agricultural planting structure on the basis of correcting the errors in previous studies on the connection and use of panel data of National Fixed-site Rural Survey in rural areas. The results show that: the higher the degree of off-farm employment, the more inclined farmers are to adjust the planting structure. The mechanism analysis shows that off-farm employment affects the adjustment of agricultural planting structure, which is affected by the adjustment effect of agricultural mechanization. Farmers with higher degree of off-farm employment are more inclined to plant crops with high level of agricultural mechanization (such as grain), which will result in the decrease of diversification index of agricultural planting structure. In the future, the government should promote the citizenization of agricultural transfer population, design support policies for grain farmers, and achieve regional balanced development of agricultural mechanization.