An increasing number of existing structures are approaching the end of their technical service life. Therefore, monitoring these structures to get the information of their health condition, which is called structural health monitoring (SHM), is becoming more significant. In SHM, acoustic emission (AE) technique shows promising features in detection, localization and characterization of damage. This technique has been applied in many fields, such as steel structures, composite structures and concrete structures. In dealing with existing concrete structures, cracks can influence the features of AE signals when they are on the way from the defect to the sensor. Therefore, it may challenge the commonly-used notion of AE monitoring. The influence of crack on AE monitoring has not been sufficiently investigated. More quantitative assessment on the crack influence on AE signals is believed to be valuable for the evaluation of accuracy and reliability of AE monitoring of existing concrete structures. In this research, the influence of crack on AE signals has been studied quantitatively. Experiments on cracks with different opening have been performed to support the formulation of the signals. The results are then used as inputs for an analytical study on source localization influenced by a crack. A three-dimensional (3D) triangulation technique has been used for source localization on a modeled cracked concrete beam. Localization error can therefore be estimated for AE sources in the presence of existing cracks. Furthermore, the influence of a crack on the amplitude of AE signals has been discussed. With the demonstrated crack-induced errors and attenuations ranges, the results can provide insights to the accuracy and reliability of AE monitoring in practical situations.