When the primary mirror position of a telescope is adjusted, its position should be measured in real time. This paper proposes an on line measuring system for the primary mirror position based on position sensors. By taking the deformation of a mirror cell into account, a algorithm to solve the primary mirror position was obtained by using the space analytic geometry method. A 1.23 m SiC mirror in our lab was used to complete the online measuring experiment for the primary mirror position. The measuring data were collected in real time by using 6 position sensors fixed at the back and later of the mirror when the mirror was turning around the rotation axis. The cell deformation was also analyzed by using finite element method. Finally, the measuring values by the sensors and the cell deformation value were taken as the systems difference to introduce the algorithm to obtain the X\Y\Z displacements, rotation angles around X\Y axis and the extension of mirror radius caused by temperature changing. Experimental results indicate that the stiffness of axial supporting mechanism is much larger than that of the lateral one. When the rotation angle of mirror is 45°, the mirror displacement in Z direction is about 20 μm, and that in X and Z directions are 146 μm and 100 μm, respectively. The accuracy of this method is verified by the test results, which provides a reference for the correction of primary mirror positions in telescopes.