In order to improve the tooth pitch measurement accuracy of the visual measurement instrument for medium and small module straighttooth cylindrical gear, the influence law of the gear base circle positioning eccentricity on the tooth pitch measurement error in the visual coordinate system was analyzed. Through theoretical analysis and simulation calculation, the sinusoidal curve model of tooth profile initial phase angle error caused by base circle positioning eccentricity was obtained. Furthermore, the influence law of base circle positioning eccentricity on tooth pitch measurement error was studied. Based on the tooth pitch measurement principle of the relative method of visual measurement instrument, the increment formula of tooth pitch measurement error was deduced, and the actual gear measurement experiment was carried out using gear visual measurement instrument. The experiment results show that for the increment model of the tooth pitch measurement error caused by base circle positioning eccentricity has high calculation accuracy, which can be used in the accuracy analysis during the research and development of gear visual measurement instrument. it is also found that when the eccentricity e≤40 μm and the positioning error Δψj>1°, the model can meet the measurement requirement of the gears with the accuracy of class5; and when the number of gear teeth z≥45, the double tooth pitch measurement method can be adopted to improve the visual measurement efficiency, which can meet the requirement of the gears with the accuracy of class5.