It is a common construction method to pour arch concrete in arch centering. The deformation law of arch centering was analyzed in process of pouring arch concrete, a new method to determine concrete cast-in-situ section lengths and corresponding formulas which based on deformation scale of arch centering were presented. According to influence line of arch centering deflection, a program used to calculate cast lengths and arch centering deformations was developed. This program could automatically calculate each segment of arch concrete based on the assigned deformation conditions. A numerical result shows that when the arch centering deformation scale was taken as the target, deformation proportion caused by each segment of concrete should be controlled on 0.5-0.6.