The invention discloses a testing tool for measuring the tooth thickness of a heavy-duty spiral bevel gear on line, which comprises a ball head, an indicator clamp, a dial indicator, a movable clamp,a vertical rod, a slide rod, and a base. Both the vertical rod and the slide rod are cylindrical straight rods, and the vertical rod is articulated with the slide rod through the movable clamp on oneside. The movable clamp, which is a tightness-adjustable clamp, is used for changing the locking position and bending angle of the clamped slide rod. The extended end of the slide rod is directly articulated with the indicator clamp, and the dial indicator, the meter of which is downward, is inserted in the horizontal indicator clamp. The meter of the dial indicator can be freely extended and retracted, and the ball head is stuck at the end of the meter. The bottom end of the vertical rod is fixedly connected to the angled base. The testing tool is a rod type testing tool, the structure is simple and light, moreover, the requirement on detection conditions is not high, above all, a workpiece does not need to be moved during measurement, and therefore the testing tool is suitable for on-line detection on the production site. Because only one person is needed to use the testing tool for detection, the testing tool can save manpower in comparison with the prior art, and thereby the production cost can be remarkably reduced.