Natural Next-to-Minimal Supersymmetric Standard Model (nNMSSM) is featured by predicting one CP-even Higgs boson satisfying $m_{h_1} lesssim 120 ,{rm GeV}$ and Higgsinos lighter than about 300 GeV, and consequently the cross section for DM-nucleon scattering in this scenario is usually quite large. We study the diphoton signal of the light Higgs boson in nNMSSM by considering the tight constraints from the latest LUX and PandaX-II experiments, and we conclude that the optimal value of the signal rate at 8 TeV LHC is greatly reduced in comparison with earlier predictions. For example, previous studies indicated that the rate may exceed $120 ,{rm fb}$ for $m_{h_1} simeq 80 ,{rm GeV}$, while it is at most $25 ,{rm fb}$ if the lightest neutralino in the scenario is fully responsible for the measured DM relic density. We also investigate the case of $m_{h_1} simeq 98 ,{rm GeV}$ which is hinted by the excesses of the LEP analysis on $Z bar{b} b$ signal and the CMS analysis on the diphoton signal. We conclude that nNMSSM can explain simultaneously the excesses at $1sigma$ level without violating any known constraints.
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