In this contribution, a donor-acceptor (D-A) copolymer PTP8, consisting of alternating benzodithiophene and thienopyrroledione with conjugated side-chains on both donor and acceptor units, was sucessfully prepared. We further investigated the effect of polymer molecular weight on polymer physicochemical properties, solar cell device performance, polymer-PCBM blend morphology, and, most importantly, polymer/PCBM blend ratio. We found that increasing the molecular weight of the donor polymer can both effectively improve the device performance and simultaneously stabilize solar cell efficiency over a wide range of polymer/PCBM blend ratios (from 1:0.5 to 1:1.0), which may lead to more thermally stable and cost-effective devices. Through intensive morphological investigation, we propose a sound morphological evolution for PTP8/PCBM blends with different molecular weights at low fullerene content.