The sensitivity of an optical system to assembly errors can greatly affect the actual image quality. The system design must be changed if its tolerance for assembly errors is too tight. In this paper, we present an automated design method for initial systems of freeform reflective optical systems that are insensitive to assembly errors. By this method, an assembly-insensitive initial system comprised of surfaces with different types (sphere, aspheric, freeform) can be obtained directly without human interaction. To illustrate the design process, an assembly-insensitive off-axis optical system that has two freeform surfaces and one spherical surface is designed, which shows the effectiveness of the proposed method.