Hygrothermal environments can degrade anisotropic conductive adhesive (ACA) joints by weakening the shear strength of adhesive interface. In this paper, the shear strength degradation model of ACA joints under hygrothermal conditions was formulated through experimental testing and theoretical modeling. The shear strength degradation data were obtained from different hygrothermal aging tests and the ACA moisture properties were characterized for the corresponding hygrothermal conditions. Theoretical models considering the hygrothermal factors of T (temperature), RH (relative humidity), and t (time), were used to fit the shear strength degradation data. It was found that the inverse exponential law was the best candidate model to predict the degradation data. The shear strength degradation model of ACA joints under hygrothermal conditions was proposed, where the relationship of the S (shear strength) and the hygrothermal factors (T, RH, and t) was expressed in an analytical model. The degradation model was validated by experiments, and the model predictions agreed well with the test results.