The present invention discloses a device for detecting axis coplanarity of orthogonal rotary shafts having a built-in intersection, wherein a front assembly rotary body is coaxially connected on a front assembly housing, the front assembly rotary body and a rear assembly housing are fixedly connected with each other, the rear assembly rotary body is mounted in the rear assembly housing, a first three dimension movement fine tuning mechanism is mounted at an end of the rear assembly rotary body, the first three dimension movement fine tuning mechanism is connected with a standard sphere through a connecting rod, the three dimension movement fine tuning mechanisms are provided in the horizontal direction of the standard sphere and below the standard sphere in vertical direction, and two non-contact displacement sensors are mounted on said two three dimension movement fine tuning mechanisms respectively. The present invention also discloses a precision measurement method for measuring axis coplanarity of the orthogonal rotary shafts having a built-in intersection by using the above device, having a feature of high measurement precision, which is adaptive to coplanarity detections for the cantilever-type orthogonal shafts having a built-in intersection and the clamp-type orthogonal shafts having a built-in intersection.