The existing low-light image enhancement methods may cause under enhancement, unbalanced brightness and blurriness. To address these shortcomings, we proposed the non-linear mapping method based on the Retinex theory (NMMR). We use an improved traditional gamma function to estimate the reflectance, and we proposed the maximum brightness channel to estimate the illumination. The main steps of this approach can be described as follows: First, we convert the image from the RGB (red, green, blue) color space to the HSV (hue, saturation, value) color space and the V channel is processed to estimate the reflectance and illumination. Then, we use a piecewise function to stretch the gray level dynamic range to achieve contrast enhancement. Finally, we use the fast pixelwise method to correct the color saturation and convert the image to the RGB color space. The experimental results show that the proposed method has lower computational complexity, the enhanced image has better objective and subjective evaluation results than other state-of-the-art methods