The invention relates to a method of distilling crude benzene by decompression method and a device thereof. Rich oil is preheated to 165-175 degrees centigrade after being subjected to heat exchange through an oil gas heat exchanger and a rich and poor oil heat exchanger; the rich oil enters in a crude benzene tower for crude benzene distillation; the crude benzene product is extracted from the tower top, and one part of the poor oil at the tower bottom is returned to the crude benzene tower as the distilling heat source after being heated by a tubular furnace while the other part of the poor oil is returned to a benzene washing tower after being subjected to heat exchanging and cooling; and the crude benzene steam escaped from the tower top enters in a crude benzene reflux groove after being cooled by the oil gas heat exchanger and a crude benzene condensing cooler, one part of the crude benzene steam is conveyed to the tower top as the back flow by a crude benzene reflux pump while the other part of the crude benzene steam enters in a crude benzene storage groove, and the crude benzene products are pumped to an oil depot. The lower part of the oil gas heat exchanger is connected with a crude benzene middle groove and is connected to a vacuum pump; the oil gas heat exchanger is connected with the crude benzene tower through the poor and rich oil heat exchanger; the bottom of the crude benzene tower is connected with the tubular furnace through a crude benzene tower circulation pump; and the tubular furnace is connected with a regeneration tower. The invention has the advantages of energy saving, emission reduction, environment protection, synergistic effect, low investment, small floor area, device reliability and flexibility and the like.