Four new copolymers P1-P4 containing the same backbone of bithiazole acceptor unit and benzodithiophene donor unit but different side chain pattern were synthesized by Pd-catalyzed Stille coupling. The effect of the side chains on backbone conformation, solubility, absorption spectra, energy levels, charge transport, blend film morphology, and photovoltaic properties of the polymers were experimentally and theoretically investigated. The planarity of the main chain increases in the order P3 < P4 < P1 approximate to P2. Upon increasing the planarity of the main chain, the polymer exhibits red-shifting of absorption maximum in film from 448 to 544 nm, upshifting of HOMO from -6.0 to -5.4 eV, downshifting of LUMO from -2.6 to -2.9 eV, and increasing of hole mobility from 4.7 x 10(-4) to 0.06 cm(2) V-1 s(-1). Upon increasing the planarity of the polymer main chain, the phase separation size in polymer:PC71BM blend increases. The polymer solar cells based on P4:PC71BM (1:1, w/w) exhibit highest power conversion efficiency of 2.54% under AM 1.5, 100 mW cm(-2), which is attributed to combination of broad absorption, high mobility, and suitable phase separation benefited from moderate planarity of the main chain.