Through the microwave-hydrothermal processing at 70 °C in 10 min, barium strontium titanate (BST) nanopowders are prepared by using butyl-titanate, barium nitrate and strontium nitrate as raw materials. Two kinds of dispersants namely polyethylene glycol 6000 and OP-10 are added, respectively. The dispersant mechanism and dosage on the crystal phase and microstructure are studied by X-ray diffraction (XRD), fourier transform infrared spectroscopy (FTIR), transmission electron microscopy (TEM). The results show that the dispersive effect of OP-10 is better than the polyethylene glycol 6000. The long chain of OP-10 can be used to disperse nanopowders through a space steric hindrance mechanism, well-dispersed BST nanopowders are obtained when mass percent ratio of dispersant (OP-10) is 5%.