Eucalyptus is widely planted as an economic forest in the humid regions of southern China due to its fast growth rate, adaptability, and high economic benefits. However, the impact of large-scale eucalyptus plantations on water resource amout and floods has become a major concern. To examine this issue, a distributed hydrological model with physical mechanisms was constructed for the Liuxihe reservoir watershed in Guangdong Province. Scenario analysis method was applied to simulate the impact of land surface pattern under different eucalyptus plantation scenes on water resource amout in the watershed. The study found that large-scale eucalyptus plantation reduces annual water resource amout compared to common tree species such as natural mixed forest. For every additional 10 000 hectares of eucalyptus plantation, the annual water resource amout can be reduced by approximately 22 million m³. Although the impact on floods is less pronounced and the flood process under different eucalyptus planting scenes are basically the same, the flood peak flow changes are relatively significant. For every 10 000 hectares of eucalyptus plantation, the flood peak flow can be increased by approximately 10 m³/s. Overall, the large-scale planting of eucalyptus aggravates the loss of water resources, increases the peak flow of floods, and raises the risk of flood and drought disaster prevention.