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Hygroscopicity-resistance of an α-starch based composite binder for dry sand molds (cores) has been studied experimentally and theoretically. Focus is placed on the relationship between the hardening structure and humidity-resistance of the composite binder. The results show that the α-starch composite binder has good humidity-resistance due to its special complex structure. SEM observations illustrate that the composite binder consists of reticular matrix and a ball- or lump-shaped reinforcement phase, and the specific property of the binding membrane with heterogeneous structure is affected by humidity to a small extent. Based on the analyses on the interplays of different ingredients in the binder at hardening, the structure model and hygroscopicity-resistant mechanisms of the hardening composite binder were further proposed. Moreover, the reasons for good humidity-resistance of the composite binder bonded sand are well explained by the humidity-resistant mechanisms.