The work aims to investigate the effects of the concentration, diameter and ultrasonic intensity of carbon nanotubes on the cold storage characteristics and heat transfer properties of nanofluids. In this paper, multi-walled carbon nanotubes with three different tube diameters of 5-12 nm, 10-20 nm and 20-30 nm and 0.05%-0.2% mass fraction were prepared as different nanofluid samples, and a test rig was set up to conduct cold storage experiments on the samples. When the mass fraction changed from 0.05% to 0.2%, the average supercooling degree of the nanofluid decreased by 0.7 ℃. And when the tube diameter changed from 20-30 nm to 5-12 nm, the average supercooling degree decreased by 64.3%; the first-order ultrasonic intensity increased the heat transfer capacity by 5.3%, and the fourth-order ultrasonic intensity increased the heat transfer capacity by 7.8%. The results show that with the increase of carbon nanotubes mass fraction and the decrease of tube diameter, the heat transfer capacity of the nanofluid increase and the supercooling degree decrease. The ultrasonic intensity can enhance the heat transfer capacity of multi-walled carbon nanotubes/water nanofluids.