Abstract: The power of the 13.5 nm (2% bandwidth) emission is one of the key parameter for the EUV source. It's important to study the temporary of the plasma in the capillary, which is useful to optimize the discharge structure and the EUV source power. In this paper, the pinch process of the plasma and the collection efficiency under different inner radiuses of the capillary were calculated theoretically. And the influence of the inner radius of the capillary on the dynamic condition of the 13.5 nm (2% bandwidth) emission was detected by a EUV monitor, which was used to analyze the influence of the inner radius of the capillary on the pinch process of the plasma. By combing the theoretical and experimental results with the design of collectors in this system, the 13.5 nm (2% bandwidth) emissions at the IF point were calculated under different inner radius of the capillary. The results show that the optimal of the power at the IF point has been got with the inner radius of the capillary 7 mm, the Xe gas pressure 7 Pa and discharge current 28 kA.