The invention relates to sugar-free yoghourt and a preparation method thereof. The sugar-free yoghourt is prepared by fermenting a dairy product, a non-carbohydrate sweetening agent, sugar alcohol, a stabilizing agent and sea backthern powder through strains, wherein the stabilizing agent is selected from a combination of pectin, gelatin, modified starch, sodium chloride and sodium pyrophosphate.The yoghourt disclosed by the invention not only solves the problem of influence on the yoghourt flavor caused by casein crystallization and yoghourt tissue state change due to Vc enrichment, strong acidity and special flavor of the sea backthern in the prior art, but also has various healthcare and nutrition actions of multiple probiotics and sea backthern. The yoghourt disclosed by the invention is suitable for being drunk by consumers incapable of absorbing the sugar, and the due sweetness of the yoghourt is kept.