Generally there are two methods for measuring the spring coefficient: tension method and harmonic oscillation method. In the traditional stretching method experiments, most indirect measurements are made by adding or subtracting weights. In traditional harmonic oscillation experiments, a photoelectric gate is used to time the period of the spring oscillator. In this article the complementary mental oxide semiconductor (CMOS) image sensor of a smartphone is primarily utilized to measure the position of the oscillator as it varies with time in the harmonic oscillation. The period and damping information are extracted from the image, and thereby the spring coefficient is obtained. After the improvement, the standard deviation and relative error of the experimental data are smaller, which are 0.0037 N/m and 0.26 percent, respectively. The physical image of the weak damping simple harmonic vibration can be observed in detail and the weak damping coefficient in air and water can be measured.