This experiment was conducted to study the effects of different levels of brewer's grains (BSG) on fermentation parameters and cellulase activity in rumen. Four Laoshan dairy goats [(40.0±2.5) kg] fixed with permanent rumen fistulas were chosen as the experiment subjects, and a 4×4 Latin square design was adopted in this study. The dairy goats were fed the diets contained different levels of BSG: the control group was fed a basal diet without BSG, and the trial groups 1, 2 and 3 were fed the diets supplemented with 10%, 15% and 20% BSG based on the dry matter, respectively. Then the below values in rumen fluid were determined in 24 h: pH, ammonia nitrogen concentration, microbial protein production, volatile fatty acid concentration and the activities of carboxymethyl cellulose, cellobiase, microcrystalline cellulose and xylanase. The results showed as follows: 1) the pH and the ammonia nitrogen concentration in all groups were within the normal range; the microbial protein production in trial group 2 was significantly higher than that in control group (P<0.01). 2) the acetic acid concentration in trial group 2 was significantly higher than that in other groups (P<0.01), and the total volatile fatty acid concentration in trial group 2 was significantly higher than that in control group (P<0.01) and trial group 1 (P<0.05). 3) From the perspective of cellulase activity, the activities of the four kinds of enzymes in trial group 2 were significantly higher than those in control group (P<0.01). In conclusion, under the experimental condition, adding 15% BSG into the diet is the most conducive to fermentation and cellulase activity in rumen.