The surge motion significantly affects the FOWT’s aerodynamic performance and the flow field around the blade. The mechanism underlying the dynamic stall of the airfoil is complex. This paper analyzes the S809 airfoil. Four coupled pitch and surge motions are produced: backward surge and downward pitch, forward surge and downward pitch, backward surge and upward pitch, and forward surge and upward pitch. Two surge frequencies are considered: fy=0.083Hz and fy = 0.121Hz. The analysis focuses on the effects of different coupled motions on the hysteresis response characteristics, the separation vortex on the airfoil surface, and load changes during the dynamic stall of the airfoil. The results indicate that the surge direction affects the dynamic stall trend of the airfoil. When the surge motion is backward, the airflow near the airfoil exerts a downward force on the boundary layer, preventing the generation of the separation vortex. A forward surge motion advances the stall, creates a large separation vortex, and results in a large hysteretic response of the flow field. The surge frequency affects the lift and drag amplitude, and the unsteady lift and drag coefficients increase with the surge frequency.