One of the focuses in arch dam health monitoring researches is to explore how to make the limited number of sensors placing in the appropriate positions.In view of the shortcomings in current research methods, a new combination algorithm called MSE-EI-MAC is proposed on the basis of the three-dimensional finite element model of concrete arch dam.In this algorithm, the MSE-EI method is employed to select all the freedom of structure and determine the candidate sensor locations; the MAC matrix and the corresponding of the maximum off-diagonal variation of matrix is employed to select the better location of sensors.It is shown in an optimal instance of 20 sensors for an arch dam considering the first six order modal vibration mode, that the sensors placement based on MSE-EI-MAC presents satisfactory accuracy of modal expansion results, linear independence and orthogonality of modal vectors, making the least of relative mean square error of the measured vibration modes, which provides better identification of mode parameters.