The invention provides an inner diameter measuring device used for measuring the inner diameter of a hole of an object, comprising an inducting unit, a data processing unit and a display unit, wherein the data processing unit connects the inducting unit and the display unit; the inducting unit comprises a multijaw detection contact which includes a plurality of measuring jaws arranged in a circle, and the end of the jaw can scatter outwards to push against the inner wall of the hole; the detection contact is provided with an angle sensor which is used for sensing the opening angle of the measuring jaws, generating sensing signals and transmitting the sensing signals to the data processing unit; and the data processing unit processes the sensing signals and utilizes the angle value of each measuring jaw for calculating so as to obtain the inner diameter of the object, and then the display unit displays the inner diameter. The invention further provides a method for measuring the inner meter. Compared with the prior art, the inner diameter measuring device and the measuring method utilize the opening angle of the measuring jaws for calculating the inner diameter of the object to be measured, therefore, the measuring result is accurate.