The purpose of this experiment was to explore the effects of yeast preparations on in vitro rumen fermentation of low-concentrate substrate, and to provide a theoretical basis for its application in ruminant nutrition. The study employed an in vitro gas production to compare the effects of four yeast preparations: active dry yeast products (ADY1 and ADY2) and yeast culture products (YC1 and YC2) using a total mixed ration (concentrate-to-roughage ratio 30:70) as the fermentation substrate. The results showed that ADY2 significantly increased ammoniacal nitrogen (NH₃-N) concentration compared to the control (P < 0.05). ADY1 significantly elevated acetic acid concentration while reducing valeric acid levels (P < 0.05). Methane production showed a numerical decrease with ADY1 and YC1 supplementation and a numerical increase with ADY2 and YC2 addition compared to control, with ADY2 and YC2 producing significantly more methane than ADY1 (P < 0.05). All yeast treatments significantly reduced lactic acid concentration relative to control (P < 0.05). These findings indicate that yeast preparations can positively influence rumen fermentation parameters, with effects varying by product type, suggesting potential for targeted applications in ruminant feeding strategies.