The invention provides an on-line monitoring method for a cable terminal on-line monitoring device. The method at least comprises the following steps: a signal detection step: a current transformer detects a current signal of a cable core of a cable terminal, and converts the current signal into a voltage signal through a resistor; a signal processing step: an amplifier amplifies the voltage signal; a data acquisition step: a CPU converts the voltage signal obtained after amplification of the amplifier into a voltage value; a data display step: a display device displays the voltage value in real time to allow staffs to be able to know the aging condition of cable terminal silica gel in real time; and a warning sending step: the CPU is internally provided with a preset voltage value; the CPU compares the voltage value with the preset voltage value; when the voltage value is larger than the preset voltage value, a clock begins to carry out timing; the clock is internally provided with afirst preset time; when the timing time is larger than the first preset time, it is judged that the cable terminal is in an aging critical condition, and the CPU sends a warning signal to the displaydevice to achieve a preventative notification purpose; and maintenance personnel can replace the cable terminal in time.