A novel two-dimensional atmospheric pressure plasma jet device is proposed for large-area processing. A finite element algorithm is adopted to obtain the optimized dimensions of the designed device based on microwave power reflection coefficient |S11| and electric field intensity. The plasma parameters of electron density and electron temperature within 0.1 ms after microwave power input were analyzed. When plasma is fully excited, is close to 3.5 × 1018 m−3, and is calculated to be 2 eV. Besides, an experimental system is established, and microwave power of 70 W can process 30 L of argon per minute, which shows a high energy efficiency. Two plasma jets with a width of 10 mm and a length of 5 mm are successfully excited. In addition, reflected power measured by the power meter is only 9.5% of incident power, which is in good agreement with simulation results.