An external cavity is built with a plane grating to narrow the linewidth of the diode bar laser, and the pumped laser with center wavelength of 766.5 nm and linewidth of 0.12 nm is obtained. After the beam shaping, the laser is focused on a potassium vapor cell center, which is 8 mm long and is filled with 600 Torr (1 Torr=133.322 Pa) helium as buffer gas. Keeping the potassium vapor cell's temperature at 190 ℃, we obtain output power of 138 mW with 769.9 nm center wavelength of potassium laser.