The current work aimed to screen the ruminal cyanide-utilizing bacteria and evaluate the influence of fresh cassava root (FCR) and pellets containing high sulfur (PELFUR) on cyanide content, kinetics of gas, in vitro degradability, and ruminal fermentation. The experiment was conducted in a Completely randomized design (CRD) for a screening of cyanide-utilizing bacteria and the dietary treatments were the level of cyanide at 0, 150, 300, and 450 ppm. A 5 × 3 factorial arrangement in a Completely randomized design was used for in vitro study. Factor A was the level of FCR at 0, 260, 350, 440, and 530 g/kg of 0.5 g dry matter (DM) substrate, and factor B was the level of PELFUR at 0, 15, and 30 g/kg DM substrate. Adding different doses of cyanide significantly affected cyanide-utilizing rumen bacterial growth (p < 0.05). Increasing the concentration of cyanide from 0 to 150 and 150 to 300 ppm, resulted in an increase in cyanide-utilizing rumen bacteria of 38.2% and 15.0%, respectively. Increasing the FCR level to more than 260 g/kg of 0.5 g substrate could increase cumulative gas production (p < 0.05), whereas increasing doses of PELFUR from 15 to 30 g/kg increased the cumulative gas production when compared with that of 0 g/kg PELFUR (p < 0.05). Cyanide concentration in rumen fluid decreased with PELFUR (p < 0.05) supplementation. Degradability of in vitro dry matter and organic matter following incubation increased at 12 and 24 h due to PELFUR supplementation with FCR and increased additionally with 15 g/kg PELFUR (p < 0.05) in 440 g/kg FCR. Proportions of the total volatile fatty acids, acetic acid (C2), propionic acid (C3), and butyric acid, as well as the ratio of C2 to C3 among supplementations with FCR (p < 0.05) were significantly different. As the proportion of FCR increased to 530 g/kg of the substrate, the volume of C3 increased by 14.6%. This is the first finding of bacteria in the rumen capable of utilizing cyanide, and cyanide might function as a nitrogen source for bacterial cell synthesis. Inclusion of FCR of 530 g/kg with 30 g/kg PELFUR could increase the cumulative gas production, the bacterial population, the in vitro degradability, the proportion of C3, and the rate of the disappearance of cyanide.