The utility model relates to an on-line detecting device for safety valves, comprising a lifting mechanism, a hydraulic system, a force sensor and two displacement sensors A or B. The displacement sensor A is composed of a coil, a copper core, an iron core and an outer shell. The coil is integrated with the outer shell and is fixed on an upright post. The inner core is connected by the iron which is arranged on the upper part and the copper which is arranged on the lower part. The iron core is connected with a lifting draw rod. The copper core is connected with a valve rod. The displacement sensor B is composed of a primary coil, a secondary coil, an iron core and an outer shell. The secondary coil is integrated with the outer shell and is fixed on the upright post. The primary coil is connected with the iron core into a whole. The upper and the lower parts of the iron core are respectively connected with the lifting draw rod and the valve rod. The detection result is accurate.