The invention discloses a method for determining improvement of thickened oil mobility of a viscosity reducer. The improvement effect of the viscosity reducer to thickened oil is determined according to the time for the thickened oil and mixed liquid of the thickened oil and the viscosity reducer which flow through pipe sections with the same distances in a viscometer; and the viscometer is placed vertically. The method comprises the following steps: adding the thickened oil in a pipe body of the viscometer, and sucking the thickened oil until the thickened oil reaches an expansion part of the viscometer and is higher than a marked line; determining the time for the thickened oil flowing to a marked line b from the marked line a and recording the time as T0; adding the mixed liquid of the thickened oil and the viscosity reducer according to different oil ratios into the pipe body of the viscometer, and then sucking the mixed liquid until the mixed liquid reaches the expansion part of the viscometer and is higher than the marked line a; and determining the time for the mixed liquid of the thickened oil and the viscosity reducer to flow to the marked line b from the marked line a, and recording the time as Tx. The method has the advantages that the test method is simple, a measurement result can visually reflect mobility of a viscosity reducer and thickened oil mixing system, and then the viscosity reducing effect of the viscosity reducer is represented.