The effective independence method (EFI) and effective independence driv ingpoint residue(EFI-DPR) were adopted to determine an optimal placement of s en sors for modal test of an arch structure. By comparing with the conventional equ al divided method (ED) through numerical and experimental analysis, the method a nd the scheme for optimal sensor placement were determined. Firstly, the modal a nalysis of the arch structure was carried out by using ANSYS, and three different optima l sensor placement methods were investigated. Then, the minimum of mean square e rror criteria and the modal assurance criterion were used to assess the appl icability of the chosen methods. Finally, the optimal placement scheme of sensor s was presented according to a real arch modal experiment. It is found that the ED method has certain rationality in the arch modal experiment, but its randomn ess is bigger. The EFI method overcome that shortcoming and the mean square error changes regularly with change of the sensor number, which provides abasis for determining the number of the optimal sensors.