Because the tethered balloon has the characteristic of floating for a long time in the sky, the control system must be of safety and reliability. Four aspects are considered including the redundancy, emergency response capability, fast replacement and electromagnetic compatibility for the design of ball control computer. The redundancy designs are conducted on the data processing, pressure detection channel and data transmission channel based on modularized network. The emergency response and electromagnetic compatibility are designed as well. Failure free operation lasting for several months has validated the high reliability and safety of the proposed control computer.