Feed additives have many advantages such as improving the immune system of animals and promoting the healthy growth of animals. However, misuse of feed additives has posed a serious threat to animal and human health as well as the environment. The rational use of feed additives is an important expression of farmers’ pro-environmental production behavior. Based on a micro-survey data of waterfowl farmers in Jiangxi, Hubei, and Yunnan provinces and applying the integrated structural equation framework of the theory of planned behavior (TPB) and the norm activation model (NAM), this paper analyzed farmers’ willingness and their behaviors of using feed additives under rule constraints from two dimensions of individual rationality and social rationality. Results show that respondents had a relatively high willingness to regulate the use of feed additives with mean value of 3.764, but their actual behavior was low with mean value of 3.216. In a decreasing order, farmers’ attitude, subjective norms, and perceived behavioral control have different significant positive effects on their willingness to use feed additives rationally. Awareness of consequence positively influences the ascription of responsibility, while ascription of responsibility and perceived benefits jointly activate personal norms. Both of their willingness and personal norms have a significant positive effect on farmers’ rational feed additive use behavior, and the effect of intention is greater. However, there is a certain contradiction between the two. Therefore, this paper suggests: widely enhancing the positive attitude of waterfowl farmers to standardize the use of feed additives, increasing the environmental responsibility of waterfowl farmers in the production process, creating an atmosphere of standardized use of feed additives, increasing policy support and subsidies, and strengthening supervision and inspection efforts.