Abstract: A simple non-orthogonal decoy state protocol with one weak decoy state is presented with a heralded single photon source (HSPS). Because the number of estimation terms in non-orthogonal encoding protocol is double of that in the Bennett-Brassard 1984 (BB 84) protocol, the passive decoy state method is unsuitable for non-orthogonal encoding protocol to decrease the implementation difficulty. Considering the imperfect extinction ratio of the practical laser, vacuum states are not prepared easily. Therefore, this paper combines passive decoy state method with active decoy state method to decrease the number of decoy state to one weak decoy state. The simulation results show that the protocol with one weak decoy state can obtain good key generation rate by the passive decoy idea and the secure transmission distance is only 2.2 km less than that of the theoretical limit of an infinite decoy state protocol. Compared with the existing non-orthogonal decoy state protocols, the proposed protocol is easier to implement.