Conducting research on the deterioration characteristics of power transformers in electrical grid measurement devices after being submerged in water is crucial for power grid companies to assess the risks and make maintenance decisions after flood disasters. Therefore, a method for predicting the equivalent deteriorated lifetime of power transformers after being submerged in water based on health index and Weibull failure rate model was proposed. That is, the health index of transformers was determined by using the fuzzy comprehensive evaluation method, and the operating state of transformers was described by using fuzzy language. The current state failure rate of transformers was determined by using the improved time-based and equipment state-based failure rate model. Furthermore, the bat whale optimization algorithm was used to improve the least squares method for segmented fitting of the Weibull failure rate bathtub curve, and the current state failure rate of transformers was mapped to the Weibull failure rate bathtub curve, thereby solving for the equivalent deteriorated lifetime of transformers after being submerged in water. Eight different types of current transformers were subjected to water immersion tests, and the test results show that current transformers deteriorate irreversibly after being submerged in water. The operating states of the transformers after being submerged in water are consistent with the evaluation results obtained by the model, thereby verifying the effectiveness of the method for predicting the deteriorated lifetime of transformers after being submerged in water.