According to the high bandwidth, low packet-loss ratio, high channel utility requirements of satellite optical switching network, an on-board assembly algorithm based on mixed threshold and round-robin scheme is proposed combining the ground optical assembly algorithm. The algorithm caches the IP packet according to the parity, and assembles the packet according to the mixed threshold and round-robin scheme. Through the scheme, the padding ratio of burst is lowered. From the results of analysis and emulation, it is found that the complexity is the same as that of the round-robin method; the padding ratio of burst data packer is less than 10-4 when the load is 0.65, and the packet loss rate is 10-6 when the load is 0.5. This algorithm meets the requirements of optical satellite communications.