Abstract: When using optical interferometry to inspect curved optical surfaces, the best technology parameters of light path were always designed in advance and the difficulty of inspection was evaluated in order to ensure the accuracy of inspection. A new way for calculating the interference fringe density was adopted to evaluate the difficulty of inspection for any curved optical surface and to determine the best position of reference spotlight source. Taking the aspheric surface for example, the radius of the best fitting sphere of aspheric surface, the max asphericity and the best position of reference spotlight source were calculated. Result shows the superiority of the new way in choosing the best condition of adjusting the light path of optical interference system. It can provide theoretical basis and analytical guidance for the design of interference inspecting system and the optimization debugging process.