The invention relates to an online leakage detection device for a steam safety valve, and belongs to the field of test field. An ultrasonic wave generator is arranged on a lower adjusting ring of the steam safety valve, an ultrasonic wave probe is arranged on a steam outlet pipeline of the steam safety valve, a signal collector and a computer where frequency spectrum/amplitude display analysis software is installed are arranged, and the signal output end of the ultrasonic wave probe is correspondingly connected with the signal input end of the computer. When the steam safety valve leaks steam, the leaked steam flows through the ultrasonic wave generator to generate ultrasonic waves at fixed frequency, and after detecting the ultrasonic waves generated by the ultrasonic wave generator, the ultrasonic wave probe outputs signals; whether the ultrasonic wave signals at fixed frequency or not are obviously enhanced on the computer is observed, or whether amplitude of the ultrasonic wave signals at fixed frequency exceeds a preset value or not is judged to safely and remotely judge whether the steam safety valve leaks or not online in real time. The online leakage detection device can be widely applied to the field of online leakage detection of steam safety valves of various specifications and sizes.