We propose a new method of decoy state quantum key distribution with a heralded single-photon source. Alice uses the parametric down-conversion to generate entangled photon-pairs, one of the pair is used as heralding photon. According to the results of the trigger detector the heralded photons are divided into trigger and non-trigger sets.The states of the photons in the trigger set are used as the signal states and the ones in the non-trigger sets are used as the decoy states. Because of the efficiency of the trigger detector, the two sets both have photons. The yield and error rate of the single-photon are estimated through the yield and error rate of the two sets. This method does not need changing the intensity of the photon source, is easy to implement. Analysis results show that this method can reach the same security distance as with a perfect single photon source; the key generation rate is increased a lot compared with the method with a heralded photon source and is approximately two thirds the rate of the three intensities decoy state quantum key distribution with a heralded photon source.