For success of modal testing

reasonable selection of number and location of sensors is the key issue.Relaxation-strategy is introduced to sequential optimization method

optimal placement of sensor

propose sequential algorithm with relaxation for modal testing sensor optimal placement. Taking the maximum off-diagonal element of the modal assurance criterion（ MAC） matrix as target function

an initial solution set is generated by accumulation sequential algorithm

element of solution of the set is relaxed several times until all elements cannot continue to relax. A suspension bridge is applied as an instance of optimal placement of sensor problem

traditional sequential algorithm is used as contrast algorithm. The results indicate that the new method is superior to traditional sequential algorithm.