It is described that the uncertainty of evaluation results of amplitude of square wave, including the courses of analysis and evaluation of uncertainty of it. Some error sources of amplitude evaluation are discussed, including the amplitude measuring error, the effect of the range of the mode of the density distribution method, and the others. The uncertainty of amplitude of square wave is given out, and several main methods of decreasing the amplitude evaluation errors and evaluation uncertainty are proposed. In an example of experiment, the analysis and uncertainty evaluation results of amplitude by using the method are presented in this paper.