The aim of this study is to investigate the relationship between the laser-induced optical breakdown energy threshold and the spectral signal changes with the spot radius. First of all, using the avalanche breakdown threshold theory calculates the threshold avalanche ionization rate η c. Combining with the theory of light intensity distribution and the relationship between pulse energy and peak power, numerical calculations have obtained the law of change of spot radius and threshold laser energy. Secondly, the variation of the spectral signal-to-noise ratio (SNR) with the laser energy is measured according the experimental measurement, and the threshold laser energy required for optical breakdown at different spot radius is obtained. The theoretical threshold model of the relationship between the threshold laser energy and the quadratic function of the spot radius obtained by numerical calculation is verified, and it is revised, the results show that the relative error between the revised theoretical threshold model and the experimental threshold model is less than 5.3%. At last, the influence of the spot radius on the spectral signal and electron density is also analyzed, it is found that both the spectral intensity and the electron density have a maximum at the spot radius of 11 μm. The results show that it is necessary to find the best spot radius during the laser induced breakdown spectroscopy (LIBS) experiment. This provides a reliable theoretical basis and experimental reference for further study of the influence of spot radius on LIBS technology measurement.