For the demand of spectral purity detecting in high-energy narrow-line width laser system, a method of spectral purity analysis by measuring the spectral energy distribution is reported and experimentally studied. The spectral purity analysis device is mainly composed of echelle grating, moveable blade, sensitive power meter,etc. The output spectrum of excimer laser system with a compound cavity is analysed by using this device. The experimental results show that the spectral purity detection can be achieved by accurately measuring the energy ratio of the spectral components. The spectral purity analysis device has the advantages of compact structure, low cost and high accuracy, which has important application in the building process of high-energy narrow-linewidth laser system.