In this work, a novel nanocomposite phase change material (PCM) has been designed to greatly enhance the thermal energy storage capacity and thermal conductivity. It is the first time that two-dimensional montmorillonite nanosheets (2D-MMT) have been used in encapsulating stearic acid (SA) latex particles thus to prepare composite phase change materials (PCM), which is featured by the very thin shell and ultra-high content of active core materials. It was prepared through the self-assembly of positively charged MMT nanosheets on the negatively SA latex surfaces under a strong electrostatic attraction, leading to the formation of a core-shell structural composite PCM. The microstructure, thermal performances and cycling stability of this 2D-MMT/SA composites have been researched via Fourier transform infrared spectroscopy (FTIR), scanning electron microscope (SEM), differential scanning calorimetry (DSC), zeta potential measurement and thermal constant analyzer. The experimental results have shown that 2D-MMT/SA composite contained an ultra-high mass fraction (> 80%) of SA due to the very thin MMT nanosheets shell, thus to have a tremendous latent heat storage capacity (184.88 J/g). In addition, the thermal conductivity of this PCM composite can reach to 159.46% of SA at most on account of the relatively high thermal conductivity of MMT nanosheets. Moreover, the protective 2D-MMT shell provided an excellent shape stability to the composite thus to restrict the leakage of PCM effectively. The composite also exhibited a stable cycling performance in the heating/cooling test. It is demonstrated that this 2D-MMT/SA composite would be of great promise for solar energy storage in sustainable energy field because of the very low cost, ultra-high latent heat storage capacity, promoted thermal conductivity, excellent structural stability and outstanding cycling performance.