A series of experiments was carried out on a pulse detonation rocket engine (PDRE) running on a liquid kerosene-oxygen mixture to investigate the indirect detonation initiation. The experiments investigating the effect of Shchelkin spiral on the deflagrationto-detonation transition (DDT) process demonstrated that all spirals were able to enhance flame acceleration to some extent, but successful DDT was achieved only when the length of spiral was increased to six times of the inner diameter of detonation tube (6D). For the model with the spiral length of 6D, the DDT run-up distance was about 0.5m (10D) and the sum of ignition delay and DDT run-up time was around 0.6 ms, which only occupied 0.6% of the whole cycle (100 ms). It implied that ignition delay and DDT run-up time were not the key factors of limiting the increase of frequency in a kerosene/oxygen PDRE. In addition, an experiment on detonation initiation by a flame jet through an orifice plate was successfully conducted on the multi-cycle PDRE. For detonation tubes with the orifice plate mounted 10 cm and 20 cm away from the thrust wall, the DDT run-up distance obtained was approximately 0.30m (6D) and 0.2m (4D). Compared with the spiral configuration, the DDT run-up distance was shortened by 40% and 60% in the two cases, respectively. The results implied that a rapid initiation of detonation could be achieved in a shorter distance with the approach of the flame jet ignition.