The invention relates to an online safety monitoring method for an electrical distance. The method comprises steps as follows: an image of a crossing point of a power transmission line and an electrified railway catenary is shot by a prearranged image collection device and is sent to a corresponding image analysis device; the image is analyzed by the image analysis device, and a monitoring distance between the power transmission line and the electrified railway catenary is obtained and is sent to a control center monitoring device at a far end; whether the monitoring distance coincide with a preset crossing point electric distance is judged by the control center monitoring device, and when the monitoring distance does not coincide with the preset crossing point electric distance, an early-warning prompt message is output. According to the online safety monitoring method for the electrical distance, real-time online monitoring can be performed on the electrical distance between the power transmission line and the electrified railway catenary.