In this paper, a novel extended interaction klystron output cavity is presented. With the help of a three-dimensional (3D) particle-in-cell (PIC) simulation, design and optimization geometry of the cavity is performed, which is composed of ladder loaded rectangular waveguide. In present work, the output continue wave (CW) power 724W is achieved, the efficiency and bandwidth reaches 7.5% and 1300MHz, which is operating at beam voltage and beam current is 16 kV and 0.6A , respectively.