Abstract: In 0.01 mol/L HCl medium, there is a max absorption peak at 350 nm for I₃^-. The association particle system of (CTMAB-I₃)_n is in violet-red color. It has a new absorption peak at 550 nm. The CTMAB concentration C_CTMAB in the range of 0.0~7.0×10^-5 mol/L is linear to the A_550nm value. Its regression equation is A_550nm=0.989×10⁴ C_CTMAB+0.0138,regression coefficient is 0.9995. The mole absorption coefficient is 1.06×10⁴ L/(mol·cm). A new spectrophotometric method has been proposed for the determination of cationic surfactant. The resonance scattering spectra show that the CTMAB⁺ and I₃^- combine to CTMA-I₃ association molecule by means of attract forces. The association molecules automatically aggregate to (I₃-CTMA)_n association particles. So that, the particles exhibit a resonance scattering peaks at 580 nm, and the system is in violet-red.