Origami is to transform a flat square sheet of paper into a finished sculpture through folding and sculpting techniques. It is widely used in many fields such as traditional art, furniture design, solar panels and medical devices. To address the problems of complex configuration of origami, uneasy folding, and the difficult process of establishing origami model, this paper proposes a digital origami representation and design optimization method with DAG (Directed Acyclic Graph) model and directional plane. Firstly, the DAG model is constructed, whose nodes and branches represent the paper states and folding behaviors respectively. Secondly, the constraint relations are defined and established between the point-line-surface geometric elements and the folding behaviors, making it feasible to conform to the paper folding process. Lastly, combined with DAG model, the folding design process, including similar folding, reasonable folding and fewer folding operations, can be optimized to improve the computational efficiency. The method provides a digital theory for origami and is validated and tested by the software Unity3d.