Laser-induced breakdown spectroscopy (LIBS) has been applied to measure spectral characteristics and for a quantitative analysis of the concentration of phosphorus (P) in compound fertilizer, an issue of great concern for the fertilizer industry. For analysis using LIBS, fertilizer samples were compressed into pellets and a pulsed Nd:YAG laser was employed to produce the plasma in air and argon (Ar) atmospheres. Using this approach, we analyzed the time evolution characteristics of the P analytical line in air and Ar atmospheres. Calibration curves were constructed for measurements under air and Ar atmospheres using reference fertilizer samples collected from fertilizer production enterprises. The measurement accuracy and detection limit of P by LIBS was improved under Ar atmosphere relative to air atmosphere. This study demonstrated that the LIBS method can be used for analyzing the concentration of P in practical applications.