Fan-wings, as a form of aircraft propulsion, have shown effectiveness for underwater propulsion. The underwater fan-wing thruster (UFT), which is composed of a fixed-wing and a cross-flow fan under the fixed-wing, can generate significate vertical force underwater. Although some experiments and simulations are carried out to analyze the UFT, the role of the fixed-wing to the UFT has not been researched in depth. In this paper, four parameters: the front opening angle, rear opening angle, upper horizontal length, and incoming flow angle are selected for the role study in four aspects, including vertical force, thrust force, torque, and efficiency. Computational fluid dynamics (CFD) simulations in different rotational speeds of the cross-flow fan and incoming flow velocities conditions are carried out for the UFTs with different parameters, which concurred with towing experiments. Self-driving experiments are also carried out for studying the actual cruise performance of the UFT with different parameters. After that, mechanisms of the effects are analyzed on many velocity magnitude cloud maps from CFD simulation. A physical model is utilized to explain some of the unique fluid phenomena. Finally, the role of the fixed-wing is summarized, and some principle on parameter setting is proposed.