We propose a concept of universal coherent source for quantum key distribution. The weak coherent pulse (WCP) and heralded single photon source (HSPS) are the most common photon sources for state-of-art quantum key distribution (QKD). However, there exists a prominent crossover between the maximal secure distance and the secure key generating rate in short and middle distance if one applies these two sources in a practical decoy state quantum key distribution. It is shown that by combining the heralded pair coherent state (HPCS) photon source and the practical decoy state method together, one can not only strengthen the maximal secure transmission distance, but also improve key generat-ing rate at short and medium distance. Moreover, the advancement in key generating is not confined in the particular protocol utilized and can be easily checked for both BB84 and SARG protocol. Finally, we clearly demonstrate how the HPCS-based decoy method works effectively and feasibly by proposing an efficient HPCS-based "1 signal+2 decoy" state method.
