The invention relates to imitated silk high-elasticity knitted underwear fabric and a preparation method thereof. The fabric is formed by interweaving fine-denier copper ammonia / nylon bi-component filament and spandex yarn, wherein the fine-denier copper ammonia / nylon bi-component filament is located on the front of the fabric and completely covers the spandex yarn. The preparation method comprises the following steps of: sequential weaving, fabric pre-sizing, pretreatment, copper ammonia dyeing, aftertreatment, nylon dyeing, soaping, soft finish, dehydration and re-sizing. The imitated silk high-elasticity knitted underwear fabric provided by the invention has smooth handfeel, brightness, full appearance, soft handfeel, antibacterial property, comfort and the like; the problems of low bursting strength and insufficient elasticity of the single copper ammonium imitated silk fabric are solved; and the problems of non-uniform blend dyeing of copper ammonium and chemical fiber, color yellowing and rough cloth cover are solved.