Laser induced breakdown spectroscopy (LIBS) technology has become an important method to detect the elemental composition and the content of corresponding elements in unknown substances due to its advantages of real-time, fast, multi-element analysis and less damage to samples. Some studies have shown that the hundred nanosecond laser pulse improves the LIBS spectral quality relative to conventional 10 nanosecond pulses, because of prolonging the action time between laser and sample while maintaining an effective breakdown threshold. By reducing the ambient air pressure (to the order of 104 Pa), both the LIBS spectral intensity and the signal-to-back ratio are significantly improved. In order to investigate the effect of low pressure on the spectral characteristics of copper alloy plasma induced by long pulse width laser (100 nanoseconds), a self-developed 80 ns pulse width Nd∶YAG laser (wavelength 1 064 nm, single pulse energy 20~200 mJ) was used as the excitation source. The sample was tin bronze (base element Cu mass percentage 92.9%, low content Fe is 0.007 8%) numbered BYG19431. The ambient pressure was changed by the sample atmosphere control system, and the spectral characteristics of Cu and Fe in the copper alloy matrix under low ambient pressure (1.01×105, 9.6×104, 9.2×104, 8.8×104 and 8.4×104 Pa) were studied, respectively. In the experiment, the repetition rate of the laser pulse is 1 Hz, and each stroke is a fresh surface (the sample position is replaced by a controllable rotating platform in the vacuum chamber), Five spectra with stable pulse energy are selected for each energy and pressure, and the average value is taken as the final experimental result under the current experimental conditions. The real-time monitoring of the laser pulse energy is performed by a transflective ratio 1∶1 beam splitter and an energy meter. It is found that the matrix element line (Cu Ⅰ 324.75 nm) has a relatively high self-absorption phenomenon at low energy (20 mJ, 40 mJ) under normal pressure. At 60 mJ, although the self-absorption effect is improved, the background intensity of the line is increased and the damage of the laser to the sample is increased. In order to further improve the spectral quality under the condition of low-spectrum background and micro-sample damage, the experimental laser energy is 20 mJ. The results show that the self-absorption effect of matrix element Cu is greatly reduced, the ratio of low-content Fe element in the sample increases, the plasma temperature increases, and the spectral line width narrows with the decrease of ambient pressure. When the gas pressure is 8.4×104 Pa, the signal-to-back ratio of matrix element copper (Cu Ⅰ 324.75 nm) and trace element iron (Fe Ⅰ 330.82 nm) increases by 5.31 and 2.43 times, the plasma temperature increases by 21.6%, and the line width of Fe Ⅰ 330.82 nm decreases from 0.29 to 0.21 nm compared with normal pressure, which improves the resolution of the LIBS element line to a certain extent.