In this paper， an efficient resonant circuit based on integrated interaction units is proposed to improve the beam-wave interaction for increasing the peak power of a Ka-band klystron to 200 kW. The integrated-unit circuit is designed with connecting two or several single-beam-wave interaction units across the cross section （each of which is typically used in a conventional single-beam klystron） based on the cascaded field structure in the rectangular gap waveguide with specific fusion boundary conditions. For the input cavity， two interaction units have been efficiently integrated to obtain the optimal absorption efficiency with the constant input power at ~35 GHz， through optimizing both the beam-loading parameters and cavity parameters. The output cavity has been 1） designed with two output ports for balancing the effect of the power extraction on the integrated circuit， and 2） optimized to deliver 200-kW peak power through injecting two pre-modulated beams. The overall interaction circuit of a Ka-band klystron is accordingly designed to produce the peak power of 202.9-kW with the efficiency of 40.2% and the maximum gain of 47 dB using particle-in-cell （PIC） simulations， when the two beams with the voltage of 45 kV and every current of 5.6 A are used to drive the klystron.