Celestial navigation has many advantages, such as strong autonomy, high precision and error does not accumulate over time, so the inertial navigation system always use astro observation to improve the precision of positioning and attitude, the full-time star tracker technology has become a research hot point in recent years. The guide star catalogue can be deduced from 2MASS star catalogue for the near-infrared system, so the exact sort of star is important. In this paper, the calculation method of star detecting ability is described. Based on this, a kind of sort method of navigation stars for full-time star tracker is deduced, which can be used on the condition of bright sky background. The new sort method of navigation stars is verified by the outside experiment.