This study was conducted to research the influences of increasing dietary energy level by replacing elephant grass with corn on growth, digestion and serum biochemical parameters of beef cattle, aimed to provide a theoretical basis for optimum supplement of energy for beef cattle fattening. Thirty healthy crossbred beef cattle at 21 months of age with body weight of (449.4±45.7) kg were randomly allocated into 3 groups with 10 heads per group. The three groups were low energy (LE) group, medium energy (ME) group and high energy (HE) group, and the corresponding contents of corn and elephant grass in dietary dry matter were 12.5%, 22.5%, 32.5% and 60.0%, 50.0%, 40.0%, respectively. The experimental period lasted for 45 days including 5-day pre-trail period and 40-day trial period. Average daily gain (ADG), feed to gain ratio (F/G) and economic benefit were calculated during the trial period; dry matter apparent digestibility was measured by total fecal collection method at the last 5 days; rumen fluid pH of was measured before morning feeding and at 2.5 and 6.0 h after morning feeding at the last 2 days; blood sample was collected to measure serum biochemical parameters at the last day. The results showed as follows:1) ADG was the highest in HE group (1.31 kg/d) and the lowest in LE group (0.96 kg/d), and that in ME group was in the middle (1.21 kg/d), while there was no significant difference among groups (P>0.05); dry matter intake and dry matter apparent digestibility showed HE group > ME group > LE group, and the differences between HE group and LE group were significant, while F/G showed the opposite change; 2) rumen fluid pH before morning feeding in HE group was the lowest, but the difference among groups was not significant (P>0.05); rumen fluid pH at 2.5 and 6.0 after morning feeding in HE group and ME group was significantly lower than in LE group (P<0.05); 3) economy benefit in LE group was the lowest among three groups, and gross profit was lower than HE group and ME group by 46.10% and 40.28%, respectively; 4) there were no significant differences of serum biochemical parameters among three groups (P>0.05). In conclusion, when elephant grass is used as the only forage in beef cattle fattening, dry matter intake, dry matter apparent digestibility, average daily gain and economic benefit are improved while F/G is reduced when increasing energy level by replacing elephant grass with corn; there are no adverse effects on rumen fermentation and animal health even though corn occupies 32.5% of diet, and the best economic benefit is obtained.