Polycyclic aromatic hydrocarbons (PAHs) are a group of persistent organic pollutants (POPs) which are mutagenic, carcinogenic and teratogenic. They are widely distributed in air, water and soil. Once PAHs enter the soil, they remain in the soil for a long time. PAHs are concentrated in the soil. They can enter the human body in many ways and pose a threat to human health. Therefore, it is necessary to monitor PAHs in soil. Now, traditional detection methods are cumbersome and time-consuming, which is not conducive to the widely rapid detection of PAHs in the contaminated sites. The method based on laser-induced fluorescence spectroscopy can quickly identify and detect organic pollutants in the soil. However, PAHs are volatile and can be degraded by ultraviolet light, so the selection of UV laser energy is very important. In this work,a 266nm laser-induced fluorescence system is established in the laboratory. Anthracene, pyrene, and phenanthrene are used to investigate the decomposition and fluorescence spectra of PAHs under different laser energies. The results showed that when the energy density of the laser changed, the peak positions of the fluorescence center did not shift, but the relative standard deviations of the maximum intensity at the fluorescence peaks of three PAHs decreased firstly and increased then. When the energy density was 8.54 mJ·cm-2, the relative standard deviations of the three PAHs in 10 spectral measurements were the largest, and the relative standard deviations of the fluorescence peak intensities of anthracene, pyrene and phenanthrene reach the minimum value at 1.72, 1.00 and 1.47 mJ·cm-2. The decomposition rates were 59.3%, 69.8% and 63.6% for anthracene, pyrene and phenanthrene at 100 s, respectively. At higher energies, three PAHs decompose rapidly. Compared with the other two PAHs, pyrene was more prone to photodegradation and thermal decomposition, and the relative standard deviation of fluorescence peak intensity was also higher than that of anthracene and phenanthrene. For anthracene, when the laser energy density was 1.72 mJ·cm-2, the decomposition rate was close to 0 at 10 s and 12.8% at 100 s, and the relative standard deviation of the fluorescence peak intensity was the lowest. When the laser energy density was reduced to 0.88 mJ·cm-2, the decomposition of anthracene in 100s was almost negligible. For pyrene, when the laser energy density dropped below 1.00 mJ·cm-2, the decomposition tended to be consistent, and the decomposition rate was 47.3%～47.4% at 100 s. For phenanthrene, when the energy density of the laser was lower than 1.47 mJ·cm-2, the decomposition rates no longer decreased, and the decomposition rates were 36.8%～38.6%. Pyrene and phenanthrene still decompose in low energy density.