In this paper, the speed control system controlled by motor speed was designed to control forward speed of underwater robot precisely. Based on the theory of double closed-loop speed control, motor control system was modeled firstly, and then regulator parameters were designed based on the engineering approaches. According to the characteristics of forward movement, the mechanical construction of sub-mini underwater robots and the comparison of various system designs, the double closed-loop regulator parameters of sub-mini underwater robots were obtained. And the propulsion system of sub-mini underwater robots was equipped with speed and current double closed-loop control. All the experimental results showed that within a certain range of motor speed, no static error could be guaranteed and the variable speed of underwater vehicle could be achieved.