Aiming at the problems of low signal-to-noise ratio and missed modal order in strain modal test of rectangular rudder surface structure， the optimal placement method of strain sensors is studied. Firstly， the dynamic model of strain sensor placement is established， and then based on the strain mode， the application of effective independence method（EI）， MinMAC method and singular value decomposition （SVD） method is studied. Experimental results of the working mode of the rudder model show that the strain response signal lacks the second-order modal information and the orthogonality of the strain mode vector is poor. After the optimization of the measured point， the second-order natural frequency and damping ratio can be accurately identified according to the strain response signal， and the orthogonality of the mode shape vector is therefore improved. Optimization results verify the necessity and effectiveness of the optimal placement for sensors.