In this paper, nano-silver sol substrate and micro-cavity fiber surface-enhanced Raman scattering (SERS) substrate were prepared by chemical methods. The micro-cavity structure was obtained by etching with hydrofluoric acid (HF). The experiment adopts wet detection. Firstly, the nano-silver sol substrate is mixed with Rhodamine 6G (R6G) to find the bare fiber micro-cavity structure with the strongest enhancement effect. Based on this structure, fiber SERS substrate coated with silver nanoparticles was prepared by using the sol self-assembly method and the self-assembly time was controlled to prepare different fiber SERS substrates (Ag/Fiber-x, where x is the self-assembly time 10, 20, 30, 40, 50 and 60 minutes). Using 10^-3 mol·L^-1 R6G as probe molecule, the Ag/Fiber-x substrate was screened to obtain Ag/Fiber-30 with the strongest enhancement effect. The SERS performances of nano-silver sol and Ag/Fiber-30 substrate were studied by detecting R6G solutions of different concentrations. The experimental results showed that under the same experimental conditions, the detection limits (LOD) of nano-silver sol substrate and Ag/Fiber-30 substrate for R6G are 10^-6 and 10^-9 mol·L^-1, respectively. The Raman intensity and concentration of the two substrates were logarithmically transformed and fitted at the Raman shift of 1 362 cm^-1. The R² of Ag/Fiber-30 substrate can reach 0.975 3, which was much higher than nano-silver sol substrate. The Raman signal reproducibility test results showed that the RSD values of two substrates at each characteristic peak are within a reasonable range, but the RSD value range of Ag/Fiber-30 substrate was small. The stability test results of two substrates showed that the comprehensive Raman strength of nano-silver sol substrate at 1 362 cm^-1 decreased by 45.90% after 35 days. The comprehensive Raman strength of Ag/Fiber-30 substrate at 1 362 cm^-1 decreased by 17.58% after 35 days. These results indicated that the Ag/Fiber-30 substrate has long-term stability. Simultaneously, the relative enhancement factor (REF) of two substrates were calculated. For R6G solution with a 10^-6 mol·L^-1concentration, the REF values of nano-silver sol substrate and Ag/Fiber-30 substrate were 3.49×10⁶ and 2.14×10⁷. It showed that for the same concentration of R6G solution, the Ag/Fiber-30 substrate has a stronger enhancement effect. It is an order of magnitude higher than the nano-silver sol substrate. By comparing the SERS performance of the two substrates, the results show that Ag/Fiber-30 substrate has higher sensitivity, better reproducibility and long-term stability. So, the fiber SERS substrate coated with silver nanoparticles has potential application value in trace detection, such as pesticide residue chemical analysis, biomedical detection and so on.