A new U-shape steel arch is proposed firstly in this paper, which is welded by three steel plates. It can both be used as a single section in steel structure, and also as a composite section when concrete cast in hybrid structure, in which the material superiority can be explored. Two kinds of finite element models are built, which are arches without clapboard along the whole structure, and with clapboards in various space such as 0.5m, 1m and 1.5m. Under fixed and hinged boundary conditions, out-plane stability of steel arch with 30 sets of different section dimensions is investigated, which includes whole instability and local buckling. Then based on the above results and referred to the formula of thin-wall with open slot, the bearing capacity formula of steel arch is made by Least-squares method, the formula can be referenced in the out-plane stability research of similar structure.