Woven fabric can provide variety application scenarios in acoustic field as flexible and light features. Folded curtain can be made from fabric, which is a good application example of woven fabric in noise control as sound adjuster in multi-function halls. Many geometric parameters can affect the sound absorption ability of folded curtain, such as folding shape, folding depth, the average distance of air cavity and folding period, etc. However, in random incidence condition, the relationship of geometry parameters and sound absorption ability is not clear in previous works. To obtain this relationship, Finite Element Method is used in this work. Because of the different folded shape, the contribution fractions of different angles are changed by the different geometry parameters. A series of three dimensional models is established with different geometric conditions. Different angles of incidence plain sound waves are introduced in three dimensional models. Numerical results show the detailed contribution fractions of different angles. A new formula that can predict sound absorption ability of folded curtain in random incidence condition can be gained.