A scheme of measuring fine constant of confocal spherical scanning interferometer is suggested. In the scheme，the time of scanning the neighboring transmission peak Δt2π and scanning phase half width of one transmission peak Δtφ are measured by the use of oscillograph，and the fine constant of confocal spherical scanning interferometer equals to the ratio of Δt2π and Δtφ . Based on the scheme，the fine constants are measured under different scanning frequency and voltage. The measuring results show that，under different scanning frequency and voltage，the measured fine constants all are close to the theory value of the confocal spherical scanning interferometer provided by the manufacturer，the present measuring scheme is reliable and feasible. Furthermore，it is analyzed in heory that the measured Δt2π and Δtφ are inversely proportional to the scanning frequency and voltage of the saw tooth wave inputting to the confocal spherical scanning interferometer.