By employing positive branch confocal unstable resonator, design of cavity parameters and experimental research on non-chain pulsed DF laser are carried out. The advantages of suppressing divergence angle and improving beam quality are shown by comparing plano-concave stable resonator with unstable resonator. To estimate the beam quality, diffraction limit magnification β is chosen as evaluating criterion, which can incarnate both the energy focusing degree and divergence angle excursion. 86.5% enveloping energy is used to define spot size, and 90-10 knife edge method is adopted to measure it. Nine sets of mirrors with different magnifications and mode volumes are tested and compared, and optimization method of unstable cavity parameters is concluded. Taking energy, divergence and diffraction limit magnification into account, the optimal parameters are M=1.89, D=40 mm. Laser beam with divergence angle of 0.74 mrad, β=1.35, laser energy of 1.86 J, and peak power of 16.4 MW is obtained at these parameters.