The invention discloses a method and device for determining a viscosity grade of diluted thickened oil, and belongs to the field of oil product transportation. The method comprises the steps of determining first proportion information, second proportion information, a first viscosity and a second viscosity; obtaining at least one third viscosity, specifically, the at least one third viscosity is obtained by carrying out viscosity measurement at least one time on the viscosity of the diluted thickened oil after the thickened oil is diluted by thin oil according to the first proportion information and the second proportion information; determining a fourth viscosity based on the first proportion information and the second proportion information as well as the first viscosity and the second viscosity through a preset diluted thickened oil viscosity model; and determining the viscosity grade of the diluted thickened oil based on the at least one third viscosity and the fourth viscosity. According to the method and the device, the viscosity grade of the diluted thickened oil is determined, so that when the viscosity grade reaches the viscosity grade which is suitable for safe transportation, it can be determined that the diluted thickened oil can be safely transported to an oil transportation destination, and therefore remote diversion and use of the diluted thickened oil can be realized.