The use of bluff body, such as a circle cylinder in vortex-induced vibration (VIV) to convert current energy into electrical energy is a relatively under-utilized power generation technology with broad application prospects. To develop current energy on a 3 dimensional and enhance the synergy of the converter, the vortex-induced vibration power generation device needs to be composed of double cylinders or multiple cylinders. At present, experts and scholars have mainly studied the VIV characteristics of double cylinders in detail, but there are few studies on the energy conversion characteristics of them. To solve this problem, based on the vibration characteristics of double cylinders with equal diameters and unequal diameters, the energy conversion characteristics of them are compared and analyzed, in pursuing of the optimal parameter combination. The results show that the energy capture power of double cylinders with diameter ratio d/D = 0.6 is higher and more stable. Choosing a double cylinder with diameter ratio d/D = 0.6 and spacing ratio G/D = 3–5 can obtain more water flow energy. When the spacing ratio G/D < 3, the double cylinder with the diameter ratio d/D = 0.4 can obtain higher energy harvesting efficiency. When the spacing ratio G/D>3, the double cylinder with the diameter ratio d/D = 0.8, cylinders can obtain a higher energy harvesting efficiency. When the reduced velocity is set in the range of Ur ∈ (4, 7), the double cylinders’ energy harvesting efficiency is higher, and the water flow energy can be converted efficiently.