An innovative united method based on principal component analysis (PCA) and the combination modal assurance criteria (CMAC) method is proposed for optimal sensor location. Firstly, using PCA, a raw data matrix is formed with the results of three optimal sensor location algorithms, and the principal components and comprehensive evaluation value are obtained, which can be used to choose candidate locations. Secondly, the MAC matrix of all combinations of the candidate locations is calculated and the points which meet the demand of minimum no-diagonal elements will be chosen. Results show that with the cantilever beam as a research object, the united method has better results than four traditional methods according to three evaluation criteria. Finally, the united method is used in the balance system of large area multi-object spectroscopic telescope (LAMOST) to get the sensor arrangement in two directions.