To overcome the shortcoming of existing contact resistance threshold detection algorithms that rely too much on prior knowledge and result in high false negative rate, a relay contact system fault detection algorithm based on interval evidential reasoning (IER) was presented. First, the prior threshold and contact resistance test data are converted into interval belief structures. Then, the prior threshold and test data are fused by IER based on the modified weight. Finally, the fused detection threshold is calculated. This proposed method considers the uncertainty contained in the contact resistance measurement model and the consistency of weights in the fusion process of IER, avoiding the shortage of adaptive ability of existing algorithms. Experimental verification was carried out and the results show that compared with the empirical threshold algorithm and the conventional IER method, the proposed algorithm has better adaptive ability and can effectively improve the fault detection accuracy of electromagnetic relay contact system. Considering the data imbalance problem, the proposed algorithm can further improve the performance of fault detection after resampling the original data.