Femto base station (BS) is proposed in the next generation cellular communication systems to enhance the indoor coverage. Many researches had verified the advantages of femto BS through numerical simulation and field tests. However, there lacks a fast approach to estimate the throughput that femto BS would bring to the users. Based on the radio architecture of fourth generation cellular systems, we propose an effective estimation model to analyze the throughput of femto BS networks. Furthermore, by considering the effects of randomly distributed interference sources, low outage probability, non-cooperated packet transmission, and Hybrid Automatic Repeat Request (HARQ) packet transmission, we will identify a closed form estimation of downlink throughput in a femto BS network. Numerical analysis is also provided to verify the results.