The variation of the monitoring effect quantity characterized the service behavior change of a concrete dam objectively reflects the working behavior variation of the dam. In order to ensure the safe operation of the dam, it is necessary to draw up the safety monitoring index of the corresponding monitoring effect quantity. Traditional method formulates the safety monitoring index under the assumption of certain failure probability, which is affected by the human factor. Aimingat the shortcomings of traditional approaches, integrated the idea of typical small probability method and analytic hierarchy process, the method of dividing the unequal range of evaluation grade attributes of concrete dam service behavior is proposed. The probability density function of monitoring effect quantity is constructed based on the principle of maximum information entropy, the method for formulating safety monitoring index of monitoring effect quantity corresponding to different grade attributes of dam behavior performance evaluation is put forward. The feasibility and effectiveness of the proposed method is verified by an engineering project.