Cascaded solid state transformer (Cascaded SST) is the core equipment in building energy internet. There is a problem that single processor cannot control faster or provide enough PWM ports for cascaded SST. This paper introduces a cooperative control processor based on DSP and FPGA, after analysis the principle of cascaded SST. Within the cooperative control processor, main algorithms are performed by the DSP, and duty cycle data is send through SPI bus. PWM modulation is performed by the FPGA based on carrier phase shift technology that achieves higher performances by the parallel architecture. At the same time, the GPIOs in the FPGA meet the number requirements of PWM ports. Experimental results in a two-level cascaded SST demonstrate that just one cooperative control processor can increase the number of PWM ports and suit the cascade module automatically, and control whole cascaded SST system, which is feasible and effective.