Disclosed are a transformer system and a control method of an air-drying device thereof. The air-drying device has a desiccant material stored therein and is connected to a transformer for removing water from an airflow entering the transformer. The control method includes the following steps. The first step is detecting a weight of the desiccant material. The next step is determining whether the weight of the desiccant material reaches a first predetermined weight. The next step is determining whether the transformer is in an air inflow state or an air outflow state, and is conducted when the weight of the desiccant material reaches a first predetermined weight. The last step is heating the desiccant material, and is conducted when the transformer is the air outflow state. Therefore, the lifetime of the transformer can be remarkably extended.