Carbon (C) is one of the key elements for alloy steel properties. In this paper, we used collinear double pulse laser induced breakdown spectroscopy to detect C in alloy steel samples under argon atmosphere in order to improve the sensitivity of detection of C content in low-carbon alloy steels. Firstly, the plasma images produced by Double pulse Laser Induced Breakdown Spectroscopy (DP-LIBS) were collected by high-speed camera to study the evolution of plasma morphology with inter-pulse delay time, then combining the spectral information obtained under double pulse to establish the optimal inter-pulse delay time which was 1900 ns for carbon. In addition, the influence of argon purge and argon chamber condition on the C spectral signal intensity was studied. Between these two situations, the argon chamber can effectively shield the effect from carbon dioxide in the air, and consequently improve the analysis accuracy of C content in alloy steel. Finally, we used the internal standard method to quantify the concentration of C in alloy steel samples. Compared with the results obtained by single pulse, the R2 of calibration curve for C element increased from 0.983 to 0.991 by double pulse, and the limit of detection (LOD) decreased from 206 to 110 μg·g-1. The collinear DP-LIBS improved the limit of detection of C content in alloy steel by 1.87 times. The results show that the appropriate inter-pulse delay time can effectively improve the collinear DP-LIBS spectral quality and instrument’s sensitivity. At the same time, the secondary excitation effect of double pulse can further effectively reduce influence of the fluctuation of experimental condition, and make the calibration model possesses better linear correlation.