The characteristics of seed injected oscillator consisting of main oscillator and power amplifier are analyzed by matrix optics theory. The main oscillator consists of two spherica mirrors of curvature -250 mm and 5 000 mm, spaced 2 375 mm apart. The power amplifying cavity consists of two spherica mirrors of curvature 400 mm and 5 600 mm, spaced 3 000 mm apart. The full beam divergence of the oscillator output is about scores of μrad, which can satisfy the requirements of MOPA technology in copper vapor laser.