The work aims to combine cultural genes with the concept of extension semantics, explore the application forms and design methods of Mongolian string instruments in modern design, and promote the dissemination of excellent ethnic culture. The cultural genes of Mongolian string instruments were analyzed. Dominant genes that were mainly composed of tangible shapes, patterns, skill genes, and intangible music and behavior genes, as well as the chimeric genes such as new materials and colors that were integrated into their development process were identified, providing a foundation for the establishment of an extensible primitive library. The primitive modeling method of extension theory was introduced. A primitive model of cultural genes was proposed. By deducing, reconstructing, decomposing, and mutating, an extensible appearance design element library of dominant genes was established. By using graphical semantic analysis, an extensible style image design element library of recessive genes was constructed, followed by the expression and design innovation of cultural genes. With morin khuur as an example, the feasibility and rationality of the research method were verified through the reconstruction, illustration, and design of explicit and implicit cultural genes using extension semantic methods. In conclusion, the extension semantic design and translation of Mongolian string instruments is an exploration of the digital protection of Mongolian traditional culture, and also provides reference for the feature extraction and innovative application of other Mongolian string instrument cultural heritage.