The effective distance of TV system in electro-optic tracker is one of the key performances, which must be calculated, analyzed and studied when the system is designed. The factors affecting the effective distance of the system are discussed, the target imaging size on TV，the luminance and contrast of targets and sky background on the imaging surface of TV are analyzed. A mathematic model of effective distance of TV under the standard atmosphere condition is established. The approach of combining the optics and signal process is capable of increasing the effective distance of TV system, the calculation formula of detection distance can be used to estimate the effective distance of the system and provide references for design.