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Side-chain engineering is an important strategy for optimizing photovoltaic properties of organic photovoltaic materials. In this work, the effect of alkylsilyl side-chain structure on the photovoltaic properties of medium bandgap conjugated polymer donors is studied by synthesizing four new polymers J70, J72, J73, and J74 on the basis of highly efficient polymer donor J71 by changing alkyl substituents of the alkylsilyl side chains of the polymers. And the photovoltaic properties of the five polymers are studied by fabricating polymer solar cells (PSCs) with the polymers as donor and an n-type organic semiconductor (n-OS) m-ITIC as acceptor. It is found that the shorter and linear alkylsilyl side chain could afford ordered molecular packing, stronger absorption coefficient, higher charge carrier mobility, thus results in higher Jsc and fill factor values in the corresponding PSCs. While the polymers with longer or branched alkyl substituents in the trialkylsilyl group show lower-lying highest occupied molecular orbital energy levels which leads to higher Voc of the PSCs. The PSCs based on J70:m-ITIC and J71:m-ITIC achieve power conversion efficiency (PCE) of 11.62 and 12.05%, respectively, which are among the top values of the PSCs reported in the literatures so far. [ABSTRACT FROM AUTHOR]