Rapid and automatic measurement of inner dimensions has taken its place in the field of precision measurement. On-machine measurement has the significant advantage of integration of processing and measurement, which can greatly improve processing efficiency and reduce costs. However, the measuring accuracy of existing on-machine measuring methods is easily affected by the intrinsic accuracy of machining or auxiliary equipment. As a consequence, these measuring methods cannot meet the demands of precision measurement. In this paper, an on-machine measuring approach based on incoherent information fusion is proposed to measure inner dimensions by using double synergetic lasers (DSLs). DSLs and the spindle of machine tools are taken as the measuring source and rotary power source respectively. Once the spindle rotates continuously, then a set of incoherent information pairs (IIPs) acquired by DSLs represent the whole feature of measured inner section. The IIPs can be correlated and utilized on the principle of the invariance of measured section. In practical measurement, neither specific measuring position as geometric center nor auxiliary equipment as angle encoder is necessary, and the final measuring accuracy is independent of the influence of machine tools. Inner diameter and roundness measurements are taken as examples to showcase the effectiveness of the proposed approach.