The invention discloses a rear-view device for an automatic dehydrator of an oil tank. The rear-view device comprises a rear-view mirror connected with a drainage pipeline, wherein a flow distribution pipe is arranged at the upper end of the rear-view device, and communicated with the drainage pipeline; one end of the drainage pipeline is communicated with the drainage opening of the dehydrator; an accommodating cavity for accommodating a medium which enters the accommodating cavity through the flow distribution pipe is arranged in the rear-view mirror; the outer wall of the accommodating cavity is a transparent wall. The rear-view mirror with a transparent view window is arranged on the drainage pipeline of the dehydrator, and water removed by the dehydrator is led to the rear-view mirror through the flow distribution pipe, so that the situation of the removed water can be observed by a user conveniently; if the removed water contains oil, an oil-water separation surface is formed in the rear-view mirror, so that defects in the dehydrator can be found by the user timely, and the problems of waste and environmental pollution caused by discharging of the oil can be prevented.