A method is described using rapid and sensitive Fourier transform near-infrared spectroscopy (FT-NIRS) for the determination of rhein-L-arginine cocrystal (argirein). By mixing different values of argirein into different proportions with rhein and arginine, we obtained 41 batches of samples to deatermine. Partial least squares (PLS) regression was selected as the analysis type and standard normal variate (SNV) and original spectra were adopted for the spectral pretreatment. The correlation coefficient (R) of the calibration model was above 0.99 and the root mean square error of predictions (RMSEP) was under 0.012. The developed model was applied to 10 batches of known samples with satisfactory results. The established method is validated and can be applied to the intrinsic quality control of synthetic products and other cocrystals.