<p>With the increasing demand for personalized products, product adaptability has become crucial for enabling user participation. Open interfaces play a key role in allowing third-party users to enhance and upgrade the existing products to meet diverse requirements. To assist designers in developing highly adaptable open interfaces while reducing design costs, this study proposes a method for open interface design and quantitative evaluation based on adaptable design principles. The method establishes modular composition by mapping functional requirements to modules. The function structure diagram and module correlation matrix define the relationships among modules in terms of physical structure connections, material connections, signal transmissions, and energy transfer. The morphology matrix combines solutions that meet specific connections to form open interface schemes. Variation degree is utilized to quantify the difference between the design scheme and the original product, thereby evaluating adaptability from the perspective of design costs. Variant degrees, including structure, reliability, and economy, are assessed to select the open interface with optimal adaptability. The proposed method is applied to the design of an open interface for a hybrid additive manufacturing (HAM) device, followed by additive and subtractive composite processing experiments. The experimental results show that the dimensional accuracy of the processed products can be improved by up to 20%, with the optimal average surface roughness (Ra) reaching 3.2&#xa0;μm, thus validating the effectiveness of the proposed method.</p>