The pulse jet air-assisted atomization nozzle has the advantages of good atomization characteristics and minimal influence of fuel physical properties on atomization performance，and therefore has good application prospects in ignited aviation heavy fuel engines. In order to investigate the atomization characteristics of multi fuels used on the pulse jet air-assisted nozzle，four pure alkanes of n-octane，n-decane，n-dodecane and n-tetradecane were selected to conduct comparative experiments. A phase Doppler particle analyzer(PDPA) was used to measure the particle size and axial velocity of spray droplets under the fuel injection pressure of 1.0MPa，the air injection pressure of 0.8MPa and the backpressure equivalent to standard atmospheric pressure. More attention was paid to the processes of the micro-atomization characteristics of different fuel jets from near-field to far-field. It is found that although the atomization performance of the four types of fuel droplets varies due to differences in physical properties，the Sauter mean diameter(SMD) at all sampling points remain below 20?m. With the development of the jets，it exhibits a non-monotonic characteristic of first separating and then coalescing，and the axial average velocity shows a monotonically increase. Further analysis reveals that the physical mechanism of droplets behind this phenomenon is the interaction between two competitions：Separation-coalescence and coalescence-bouncing-evaporation.