Initial configuration is important for optical system design, especially for systems with high requirements in specifications. The number and complexity of optical surfaces needed in an optical system largely depend on the choice of the initial configuration. It is essential to achieve a good initial configuration with minimum aberrations before using complicated surfaces. We propose an initial configuration design method for off-axis reflective optical systems. With this method, a good initial configuration with conic surfaces can be obtained directly. Nodal aberration theory and matrix method are used to calculate the aberration coefficients for the off-axis reflective optical system. Genetic algorithm is applied to search for the initial configuration with minimum aberrations. The validity of this design method is demonstrated by designing an off-axis three-mirror optical system.