A multi-cavity passive nanosecond laser pulse stretching system was designed. The system could reduce laser peak powers and avoid the laser induced plasma spark, so that it reduces background interference and improves the signal to noise ratio in combustion diagnostics. The theoretical model of the pulse-stretching process for an arbitrary number of optical ring cavities was introduced,and several key parameters that affect on pulse width, beam-splitter reflectivity, cavity length and total number of optical cavities were analyzed. Finally,the designed multi-cavity laser pulse stretcher was used in broadening the Nd:YAG laser harmonic pulse width.Results indicate that a 7.8 ns input laser pulse has been converted into approximately the output laser pulses in 17 ns, 35 ns and 72 ns, respectively.Furthermore, the laser peak power has been reduced to 9% of the original laser peak power, meanwhile,it maintains the good beam quality. The stretcher was also used in the spontaneous Raman scattering experiments, and results show it well resolves the laser induced breakdown spectroscopy of the Raman signal and achieves good results.