To change row spacing is a common agricultural practice to adjust plant density to enhance plant growth and development. Here we examined the fine root (≤1.0 mm in diameter) distribution and morphological characteristics of switchgrass (Panicum virgatum L.) in the 7th growth year under three row spacing settings (20 cm, 40 cm and 60 cm) on a terrace field in the semi-arid Loess Plateau region. Roots from 0 to 150 cm soil layers were utilized to evaluate the root biomass, distribution and morphological traits in April and October 2016. Results showed that fine root biomass (FRB) mainly distributed in the 0–40 cm soil layer, and it was higher under the 20 cm and 40 cm than that under the 60 cm row spacing treatment. FRB in October was lower than in April under all three row spacings. Compared with 20 cm and 60 cm row spacing, root surface area, root length density and specific root length under 40 cm row spacing were higher, while root diameter was smaller. These conferred switchgrass with larger soil exploration and root-soil interface. It implied that switchgrass grown under 40 cm row spacing had stronger ability on exploiting soil resources in the semi-arid area.