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Soil respiration (SR) in crop field is affected by environmental factors, agronomic practices and crop types. To clarify how planting density affects the SR dynamics in switchgrass (Panicum virgatum L.) field on the semi-arid Loess Plateau, this research investigated diurnal and seasonal changes of soil respiration rate (R-S) under three different row-spacing treatments (20, 40 and 60 cm) in the fourth growing year of switchgrass. Results showed that R-S presented a pronounced seasonality under all row-spacing treatments. The highest daily average R-S values appeared in August, while the lowest (P < 0.05) were in September for each row-spacing. Diurnal variations of R-S exhibited single-peak curves in each month. Daily average R-S increased significantly as row-spacing enlarged during May and August but there was no significant difference among row-spacing treatments in September. Soil water storage in the depth of 0-100 cm had no significant difference (P > 0.05) among the row-spacing treatments, and similar results were found for soil temperature in 0-15 cm soil depth. Soil respiration temperature sensitivity (Q(10)) values were 1.0-3.7 during the growing months, which were strongly correlated with air temperature in May and June and the soil temperature at 15 cm depth in August. Higher aboveground biomass production and lower R-S in most growth months indicated that 20 cm row-spacing treatment was beneficial for increasing the carbon fixation in switchgrass field. Results also implied that it is necessary to take into account the influence of phenology and root growth of switchgrass on soil respiration for accurately evaluating the carbon cycle in the region.