Abstract: Solar sail is one kind of novel spacecraft that can be propelled by sunlight pressure and does not have to carry a large amount of fuel. Therefore, solar sail can be used in interplanetary navigation and deep space exploration. Especially, logarithmic spiral trajectories of solar sail appear to be an attractive option for interplanetary navigation. During the mission, the attitude angle between the solar sail and sunlight must be fixed. However, the conventional symmetrical plat plate model cannot passively keep such attitude to satisfy the requirement. In this paper, an unsymmetrical sail design is discussed to solve the aforementioned problem, and the attitude-orbital coupling dynamics model is employed to do the simulation. The result shows that attitude passive stability on spiral trajectories can be achieved by the design of unsymmetrical sail on the condition that perturbation is slight.