A total internal reflection system based on the weak value amplification principle is set up for the precise measurement of the thickness of an ultra-thin film. In this system, the film thickness is derived from the change of the double-peak pointer caused by the effective refractive index of the film, which is correlated to its thickness. The sensitivity and resolution of this system reached 2727.21 nm/RIU and 7.2×10-6 R I U, respectively, determined by using a sodium chloride solution with a refractive index of 1.331911. The growth process of TA/Fe(III) assembled films with thicknesses in the few nanometers range is monitored using the as-set-up system, and the experimental results are consistent with a theoretical calculation based on the Maxwell Garnett effective medium. Additionally, we theoretically calculated the detection limit for the thickness measurement of the film as 22 pm. We clearly provide a potential method for the precise measurement of the thickness of an ultra-thin film.