Software Defined Satellite Network (SDSN) is a novel framework which brings Software Defined Network (SDN) technologies in the satellite networks. It has great potential to achieve effective and flexible management in the satellite networks. However, the frequent handovers will lead to an increase in the flow table size in SDSN. Due to the limited flow table space, a lot of flows will be dropped if the flow table is full during the handover. This is a burning issue to be solved for mobility management in SDSN. In this paper, we propose a heuristic Timeout Strategy-based Mobility Management algorithm for SDSN, named TSMM. TSMM aims to reduce the drop-flows during handover by considering two key points, the limited flow table space and satellite link handover. We implement TSMM mechanism and conduct contrast experiments. The experimental results verify the good performance in terms of transmission quality, an 8.2%-9.9% decrease in drop-flow rate, and a 6.9%–11.18% decrease in flow table size during the handover.