In order to explore an effective and simple technology for the preparation of carboxylated cellulose nanocrystals (CCN), we prepared CCN with microcrystalline cellulose (MCC) as raw material and ammonium persulfate (APS) as oxidant via one-pot tandem reactions. The effects of time, ammonium persulfate mole, temperature and their interactions on the yield of carboxylated cellulose nanocrystals were optimized with response surface methodology. Morphology and the particle size distribution, crystallinity, structure and thermal properties of raw material and CCN were analyzed by transmission electron microscopy (TEM), laser particle analyzer, X-ray diffraction (XRD), fourier transformation infrared spectroscopy (FTIR) and thermogravimetric analysis (TGA). The results showed that the interaction between time and temperature,time and ammonium persulfate mole was better than that of ammonium persulfate mole and temperature on the yield of CCN. With the optimization, when time, ammonium persulfate mole and temperature were 204min, 2mol/L and 62℃, the yield of CCN would reach 46.41% and accorded well with the theoretical prediction value of 46.93 %, indicating that the designed model was reliable. CCN presented a uniform distribution of rod-like shape with the diameter of 10 to 30nm and the length of 50 to 200nm, and the Z-average size of CCN was 96.92nm. The peak of C=O appeared at 1731cm-1, indicating that H2O2 produced by the decomposition of ammonium persulfate selectively oxidized the hydroxymethyl groups of C6 into carboxyl. CCN was characteristic of the cellulose I crystal form, with the crystallinity of 78.35%. CCN had a lower thermal stability than MCC.