The beam quality of no-chain pulsed HF laser cavity is improved by optimizing a positive branch confocal unstable resonator. Cavity structures of five different parameters are designed with different cavity magnifications and laser mode volume diameters. The effect of different cavity structure parameters on laser beam quality and laser pulse energy are studied by using the diffraction limit magnification based on the measuring of the focusing spots. The experimental result shows that diffraction limit magnification of 2.3 can be reached with the cavity magnification larger than 2.5. In order to get a better experimental result, the optimum diameter of mode volume is controlled within 80% of the gain cross section, and a better laser beam quality is obtained under the discharge electrode structure in the experiment.