The spectral line C I 247.86 nm which is widely used to analyze unburned carbon in fly ash by laser-induced breakdown spectroscopy (LIBS) shows strong spectrum interference. Another spectral line C I 193.09 nm located in deep ultraviolet area is frequently used to avoid the strong interference. However, it is strongly absorbed by oxygen in air, which affects the accuracy of quantitative analysis. In order to improve the performance of quantitative analysis by using C I 193.09 nm line, the spectrometer is filled with argon and the argon environment is formed in the region where the plasma emerges. The quantitative analysis results of unburned carbon in air and argon atmosphere by using C I 193.09 nm line are compared. The results indicate that the line intensity, signal- to- noise ratio, repeated measurement precision and limit of detection are improved significantly. Meanwhile, the absolute error between predicted concentration and actual concentration for two test samples is reduced to 0.02% and 0.42% (mass fraction), respectively in argon atmospheric environment, and the limit of detection is also reduced to a relatively low level of 0.37% (mass fraction).