To achieve high-precision cone inner surface measurement, it is necessary to identify the cause of error and analyze the influence of the error on the measurement result. By building the mathematical model in the cylindrical coordinate system, the formulas of offset error and angle error are derived. The result shows that the optical path difference is caused obviously by offset error and angle error of cone mirror. When the offset is extremely small, the optical path difference changes linearly with the offset. The optical path is very sensitive to angle error. To ensure the measurement accuracy, the apex angle of cone mirror must be strictly controlled.