A series of ultra‐uniform gold spherical nanoparticles with different sizes were synthesized using gold chloride acid as precursor ,ascorbic acid as reductant and sodium citrate hydrate as surfactant .The prepared Au nanoparticles were characterized by scanning electron microscope (SEM) and UV‐visible spectroscopy .The results showed that the absorption peak of UV‐Vis spectroscopy red‐shifted along with size increasing of the nanoparticles and finally appeared a quadrupole peak .To further ex‐plore the mechanism of surface enhanced Raman spectroscopy (SERS) effect and optimize the sensitivity ,SERS on Au nanopar‐ticles with different sizes were measured using Rhodamine 6G (R6G) as probe molecule .We found the SERS signals of R6G on the Au nanoaprtciles were highly size dependent .When the particles sizes are close to ~120 nm ,it will generate the highest en‐hancement ,the enhancement factor is about 1.1 × 107 .The 3D‐FDTD simulation results correlated with the experimental data very w ell .%以氯金酸为原料，抗坏血酸为还原剂，柠檬酸钠为保护剂，用化学还原（种子生长）法制备了不同粒径、超均匀的球形金纳米粒子溶胶，并通过紫外可见吸收光谱（UV‐Vis）和扫描电子显微镜（SEM）进行表征。结果表明，随着金纳米粒子粒径的增大，其UV‐Vis光谱中的吸收峰发生红移并出现四极峰。为进一步研究金纳米粒子表面增强拉曼散射（SERS）效应的作用机理并优化其灵敏度，我们以罗丹明6G（R6G）为探针分子，对不同粒径的金纳米粒子进行SERS表征，发现R6G的SERS信号随着金纳米粒子的增大先增强后减弱。当金纳米粒子的平均粒径达到120nm时，产生最强SERS信号增强，增强因子约为1.1×107。三维时域有限差分法（3D‐FDTD）理论模拟纳米粒子阵列电磁场分布结果与实验数据的趋势一致。
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