Dramatic progress in nanotechnology has enabled nano-machines to perform basic functions of communication. To interconnect nano-machines, molecular communication is one of the most promising communication mechanisms, in which molecules are used to deliver information. However, compared to classical communication schemes, the reliability of molecular communication is often criticized due to the slow diffusion process and the inter-symbol interference (ISI). In this paper, we propose a low complexity receiver design for diffusion-based molecular communication to solve the ISI problem. We first derive the optimal receiver and then use the Poisson distribution to approximate the Poisson binomial distribution in order to obtain a receiver with lower complexity. The simulation results confirm the near-optimal performance of the proposed receiver design.