Improving farmers’ usage of organic fertilizer is critical for the green and high-quality development of China’s agriculture. Based on 492 mango farmers’ survey data in Hainan Province, this paper uses an endogenous switching regression (ESR)model, empirically analyzes the impact of agricultural extension services on farmers’ willingness to use organic fertilizer, and further investigates the mediating role of ecological cognition and the moderating role of neighborhood effect in the influence mechanism. Results show agricultural extension services have a significant positive effect on farmers’ willingness to use organic fertilizer, ecological cognition has a partial mediating effect in the influence mechanism, which accounts for 17.84% of the total effect. The neighborhood effect has a positive moderating effect in the influence mechanism of ecological cognition on farmers’ willingness to use organic fertilizer. These results imply that agricultural extension services play a significant role in China’s sustainable agricultural development and by improving their awareness and taking advantage of the neighborhood effect, we can stimulate farmers’ willingness to green production. The study also puts forward policy recommendations on further promoting farmers’ use of organic fertilizer.