Quadrotor AUV is a new and promising vehicle in applications due to its special advantages. However, the problem of dynamic modeling of quadrotor AUV has not been investigated yet. In this study, a new dynamic modeling method for a quadrotor AUV is proposed by using a novel CFD simulation to calculate the hydrodynamic coefficients. Star CCM + is used to simulate the uniform and variable motion of each degree of freedom of a quadrotor AUV to decouple original high coupling dynamic model. Through the simulation of the AUV and the propeller motion, the main hydrodynamic coefficients are calculated, while the mathematical relationship between the rotation velocity and the thrust/torque is concluded. Specifically, the static mesh and dynamic mesh models are used to simulate the uniform and variable motion of the quadrotor AUV respectively. The calculation of added mass and drag coefficient does not interfere with each other, leading to accurate results. A numerical method is also proposed to separate the linear and nonlinear parts of the total drag coefficients. Then a six DOF dynamic model of a quadrotor AUV is established. The proposed CFD simulation also provides a reliable, repeatable and low-cost method for dynamic modeling of underwater vehicles. © 2021 Elsevier Ltd