Micro-nano structure with unique optical properties are commonly used in micro-nano optics to achieve luminescence enhancement of fluorescent substance. In order to improve the luminous efficiency of quantum dots (QDs), a dimer structure composed of two silicon nanospheres of different sizes is proposed. By utilising the finite-difference time-domain (FDTD) method, the enhancement in the quantum yield and fluorescence excitation rate are investigated to illustrate the fluorescence enhancement effect of the silicon nanospheres dimer. Results show that the luminous intensity of CdSe QDs can be greatly enhanced by dimers composed of two silicon nanospheres of different sizes. When the two silicon nanospheres have smaller diameter and smaller gap, the quantum yield and fluorescence excitation rate of the QDs can be enhanced more. In particular, when the diameter of the two silicon nanospheres is 100 nm and the gap is 10 nm, the fluorescence intensity of the CdSe quantum dot can be enhanced by about 209 times. The results are of certain guiding significance for the design and development of high-performance quantum dot photoluminescence devices.