Massive multiple-input multiple-output (massive MIMO) system or so-called large scale antenna system (LSAS) has been identified as one of the promising candidates for 5G wireless communications systems. In general, a massive MIMO architecture can be split into two main parts, the massive MIMO baseband digital platform and the massive MIMO radio platform. Although several studies have been already performed on the achievable theoretical results of the technology and associated impairments, fewer has been done so far in the area of real implementation complexity analysis, reduced cost research prototype, hardware and software required functional blocks and interactions. The object of this paper is to introduce a low-cost scalable massive MIMO research platform with flexible co-operated hardware (HW) and software (SW) baseband processing. The proposed solution plays an important role in exploring the optimal configuration of a massive MIMO baseband digital system, processing algorithm complexity reduction, procedure and protocols definition towards a high performance software-defined massive MIMO system. The experimental HW and SW results of the key processing units preliminarily verify the feasibility and achievable performance of the proposed flexible and scalable massive MIMO research platform, which was built with a co-operated general purpose processor (GPP) workstation and a middle-range field programmable gate array (FPGA) evaluation board.