In order to improve the maneuverability and stability of underwater robots, a novel underwater robot driven by quadrotor and undulating fin was developed and its attitude control algorithm was proposed. The kinematics and dynamics model of cooperative propulsion by the propellers and the fin was established. The resistance coefficient was obtained by fluid numerical simulation. And the six-degree-of-freedom dynamic model of the robot was constructed. The attitude control strategy using hybrid drive of propellers and fin was proposed. A four degrees of freedom cascade proportional-integral-derivative controller and a control allocation algorithm were designed. The simulation model was established in MATLAB/Simulink software, in which pose curves in fixed depth cruise motion, heave motion and pivot steering motion were simulated and analysed. In addition, the influence of random interference on attitude control was simulated. Experimental results show that the robot has good attitude control performance. At the maximum undulating frequency, the attitude angle error of the prototype is less than ±4°; and the depth error is less than ±5 cm. The simulation and experimental results verify the feasibility of the novel underwater robot and the attitude control algorithm of propeller-fin cooperative propulsion.