The disclosure is an irregular object length measuring instrument, comprising a first fixing seat, at least two mounting flanges are provided on an edge of the first fixing seat, and a notch is provided on the first fixing seat, a shaft sleeve, the shaft sleeve is mounted on the mounting flange, a main board, a battery is installed on the main board, a gear and an angle sensor are installed on the main board, and the angle sensor is installed on the gear, a second fixing seat, an edge of the second fixing seat is provided with a matching flange that cooperates with the mounting flange, the second fixing seat is installed on the first fixing seat, and the main board is fixedly installed on the first fixing seat or the second fixing seat, a swivel, an outer side of the swivel is provided with a ring groove.