The quantum key generation efficiency and the transmission distances of two kinds of point-to-multi-point quantum key distribution (QKD) system are discussed. How to select the devices’ parameters is considered detailedly when designing a multi-user QKD system. Decoy state techniques can enhance the security of point-to-multi-point QKD systems, for example, the transmission distance is extended two times and the quantum key generation efficiency is improved a factor of ten. Based on point-to-multi-point QKD systems, a protocol which can realize secret key sharing between two arbitrary receivers is proposed.