Herein, the substrate comprising carbon nanotubes and silver nanoparticles (CNTs/Ag₁) was first prepared using the one-step method, and then CNTs/Ag₁ was decorated by the additional Ag nanoparticle sol (Ag₂) to constitute the final composite structural substrate (CNTs/Ag₁/Ag₂). The corresponding electric field distributions and the enhancement factors were calculated using the three-dimensional finite difference time domain (3D-FDTD) algorithm. Experimental results of Raman test show that the detection limit is 10 ^-15 mol/L for rhodamine 6G (R6G). Quantitative detection of R6G with a low-concentration of 10 ^-15-10 ^-10 mol/L is realized using the normalized ratio k. The relative standard deviation (RSD) value of k is as low as half of the RSD of Raman intensity. Moreover, quantitative detection of crystal violet (CV) solution with a concentration of 10 ^-9-10 ^-6 mol/L is achieved. The linearity-fitting degrees of all k values of two probe molecules are larger than 99% at different concentrations.