In order to improve the accuracy of alt-alt photoelectric telescope, and meet the requirements of technology index, the theory and measurement of sighting error were discussed based on the measuring theory of alt-az telescope. Two programs of sighting error measurement were generated. The researching telescope can not work in reversed position of telescope. Sighting error was measured. The result is 4′1″ and the standard deviation of this method is 9″. Compared with the result of basic parameter model regression, it is found out that the measurement result contains the sighting axis deviation (1′4″, 2′4″) because of gravity deformation. A simultaneous determination measurement method for gravity deformation deviation and sighting error is proposed.