The adjustment of planting structure is an indispensable way to deepen the structural reform of agricultural supply side,but the excessive adjustment to follow the trend will bring great challenges to China’s foodstuff security and food security.In this paper,Bootstrap intermediary test and instrumental variable method were used to verify the herding effect and its formation mechanism in 467 maize farmers in Liaoning Province.The results show that the herding effect exists in the adjustment of farmers’ planting structure,which is further expounded by the facts that farmers tend to imitate others’ decision-making in the group,and kinship proves to be more influential than geo-relationship in decision-making.The formation mechanism of herding effect in the adjustment mainly lies in the learning imitation and risk sharing among farmers,which is more manifest among small-scale farmers than large-scale farmers.Therefore,it is proposed to complement the formal system with the herding effect,to promote the optimization of planting structure,to emphasize the leading role of new business entities to small farmers,and to avoid excessive adjustment.