Abstract: On the basis of analysis of the online working principle of testing system of the Mark line, the paper emphatically discussed the realization scheme of the online testing system of the Mark line and its design circuit.This circuit of the testing system is simple and reliable and can guarantee that when the online speed of gravure press works with high speed of 120 metre/minute, the control of the chromatography deviation of the relative standard colour of all kinds does not exceed ±0.2 mm.