Abstract: Measurement error from the photoelectric theodolite is the main error in the ballistic trajectory measurement. In order to measure ballistic trajectory accurately, the measurement precision of the photoelectric theodolite should be investigated. The measurement accuracy of the photoelectric theodolite is decided based on measurement requirement of the ballistic trajectory. Two simple equations were derived mathematically, one expression for positioning error of the ballistic trajectory, angle error of the photoelectric theodolite, ranging error, target slant range and height when one instrument is used, another expression for positioning error of the ballistic trajectory, angel error of the photoelectric theodolite, target slant range and intersection angle when two symmetrical intersection instruments are used. The two equations provide a theoretical base for designing precision of the photoelectric theodolite.