In order to improve the accuracy of transmission line image and video online monitoring equipment analysis resulting in low illumination environment, a low illumination image enhancement method based on the Retinex theory is proposed. First, an improved homomorphic filtering algorithm is used to enhance the RGB component of a low illumination image. Then, the image is converted into the HSV color space. The multi-scale Retinex algorithm is improved to enhance the image. The Gaussian function is replaced by a bilateral filter function as the surrounding function of the Retinex algorithm. The color recovery function is introduced to restore the image color. Finally, the image is converted to the RGB space to obtain the enhanced transmission line image. The simulation results show that this method can effectively improve the contrast, clarity and information entropy of actual low illumination transmission line images. It can also be applied in ice cover warning and foreign body recognition. This work is supported by the Science and Technology Project of State Grid Corporation of China (No. SGITG- 2018ZXCG-FF).