The equipment based on confocal principle is a kind of new setup, which can fast and automatically measure center thickness of lens without contact of lens surfaces. We investigate the spectral bandwidth of emergent light in order to obtain the factors influencing the spectrum bandwidth and the measurement precision. The result shows that the numerical aperture on image side and the confocal hole radius are the main factors. When the numerical aperture is larger, the spectral bandwidth is smaller and the measurement precision is higher. What is more, when the hole radius is smaller, the spectral bandwidth is smaller and the measurement precision is higher, too. Then, we adopt the result to guide the design and simulation, and we get a better numerical aperture and confocal hole radius, which can realize the precision of the measurement system.