The error caused by Finite Element Analysis(FEA) in the mode analysis of a reflective mirror set was evaluated quantitatively and identified by an experiment. The influence of source factors, arithmetic error, material parameter error and the simplification of the bolt connection on the FEA results were introduced in detail.It shows that the arithmetic error is less than one percent; the deliver factors of the material parameter errors can be calculated by the formula and test calculation dada, respectively,and the participation factors of the material parameter errors are estimated by the modal plot; the error caused by the simplification of the bolt connection is calculated by the contrast of the results of the simplification finite element model and the detailed finite element model. The results show that the FEA error would be 10% if the bolt connection is simplified.However, if the bolt connection is considered in detail,the FEA error would be decreased to 5%. The experiment results prove that the error analysis results are correct. Moreover,the trade method of the bolt connection is the key factor in the mode analysis of the reflective mirror set and it can’t be ignored apparently in the high precision analysis. It concludes that the quantitative error valuation method can be applied to the other cases, such as gravity transformation analysis, dynamic response analysis and thermal analysis.