Well-dispersed Au colloids were prepared from AuCl3 and KBH4 under the protection of polyvinylpyrrolidone (PVP) in the ultrasonic field. The Au colloids were characterized by means of transmission electron microscope (TEM), X-ray diffraction (XRD) and spectrophotometer. The results indicate that Au nanoparticles are spherical or axiolitic and the grain diameter is about 25 nm. The reductant dosage has a great effect on the UV-vis absorption spectrum of Au colloids. When the dosage of KBH4 is lower than 1 mL, the maximum absorption peak is in 530 nm and the peak is low and broad. When the dosage of KBH4 is over 2 mL, the maximum absorption peak has a large blue shift to 512 -514 nm. The peak is strong and broad. When the dosage of KBH4 is over 5 mL, the absorbency graphs are alike. When the dosage of KBH4 is over 8 mL the maximum absorption peaks are in 510 nm around. The peak is strong, sharp and narrow, so the Au colloids are well-dispersed. The ultrasonic field distribution, power and temperature have a great effect on the absorbency graphs of Au colloids. The peaks are weak and the maximum peaks are in 510 nm around.