The present invention discloses a method for measuring absolute value of non-linear error and an apparatus thereof. The method comprises: placing N reflecting plates jointed together at the sample port of the optical measuring instrument at the same time, wherein each of reflecting plate has a same covering area at the sample port; placing an aperture along light paths of the optical measuring instrument; adjusting the number of reflecting plates as used according to a position in the measuring range of the optical measuring instrument where the non-linear error is required to be measured; following every adjustment, acquiring the output results when the adjusted reflecting plates are placed at the sample port; performing a computation processing for non-linear error to the output results; and acquiring the non-linear error of the output results of the optical measuring instrument.