The invention discloses an arc measurement structure of a clamp type online roll gap instrument. The arc measurement structure comprises a box body as well as an arc-alignment plate, a connecting component and a tilt angle sensor which are connected onto the box body; the inner side surface of the arc-alignment plate is connected with the connecting component, and is attached to one side of the box body; the connecting component is disposed on a mounting guide seat in the box body; the outer side surface of the arc-alignment plate is an arc plate working surface and is used for contacting continuous casting work rolls; the tilt angle sensor is disposed on the connecting component and is used for recording the angle of the tilt rotation of the arc plate working surface when the arc plate working surface contacts the work rolls; and a plurality of movable guiding grooves and a plurality of connecting holes are formed in the connecting component. According to the arc measurement structureof the clamp type online roll gap instrument of the invention, the arc-alignment precision detection problem of a casting machine is solved based on the reasonable detection precision and the repeatability of detection results of high frequency, and therefore, the management level of the casting machine can be improved under a condition that the operation rate of the casting machine is not affected, and manual operation and intervention are not required, and batched casting blank quality defects and fan-shaped segment abnormal damage caused by the lack of arc alignment precision can be prevented.