Abstract: In view of the stereoscopic distribution characteristics of small cell networks, a 3-D stochastic geometric model for small cell networks in time division duplex (TDD) scenario is proposed in this paper and the performance analysis is carried out. First, based on the spatial distribution characteristics of small base stations in 3-D PPP model, we derive the mathematical expressions of the uplink and downlink coverage probability for the small cell networks in time division duplex system. Then, based on the results of coverage probability and the definition of spectral efficiency, the general expressions of average spectral efficiency for the dynamic TDD small cell networks with mix-interference of uplink and downlink are deduced respectively. The simulation results not only validate the rationality of the proposed model in this paper, but also show that the spectrum efficiency of the network decreases with the increase of downlink cell proportion and power control factor. In addition, the indoor environment will also have an impact on the network coverage performance.