A tandem structure is decisive for improving the efficiency and stability of thin film silicon solar cells, however, the current mismatching between two sub cells limits the efficiency improvement. In order to improve the current matching, a suitable material is required as an interlayer. The matching relationship between refractive index and thickness of the interlayer material is obtained through optical design and theoretical calculation: the selection ranges of refractive index n and thickness d are 1.59~3.1 and 125/n~175/n nm, respectively; the optimum thickness d is 150/n nm. The optimal refractive index and thickness of the interlayer material to maximize the current matching are determined to be 1.59 and 94.3 nm. The results provide a guidance for experimental design from the aspect of optical properties of the interlayer in tandem solar cells.