Agricultural supply chain finance effectively alleviates the problem of farmers’ credit constraints and realizes the commercialization and sustainability of agriculture. However, the rapid spread of farmers’ credit default behavior will seriously affect the stability of the agricultural supply chain and the wide application of agricultural supply chain finance. To prevent the rapid spread of farmers’ credit default behavior, the diffusion mechanism of farmers’ default behavior is studied with network evolutionary game. The results indicate the following: (a) When some farmers choose the default strategy because their default income is greater than their performance income, the default behavior in the small‐world network will not spread to the whole network. However, in the scale‐free network, when the default rate of return exceeds a certain threshold, it will lead to the spread of default behavior throughout the network. (b) In the small‐world network, the spread of default will be inhibited to some extent with the increase of extra returns. However, the effect of raising extra returns on the spread of default behavior is not obvious in the scale‐free network.