Statistical performance evaluation based on available ratio of antijam antenna array has been considered a common viewpoint for GNSS evaluation. But in some aspects, the traditional evaluation method has several deficiencies as follows: the first is the hypothesis of independent uniform distribution of signal DOA, which is not in accordance with the navigation constellation; the second is the single signal available ratio which is unable to meet the navigation requirements. Aiming at these two problems, the research extended the average available ratio of single signal to the average solution probability and navigation service availability which are associated with the navigation solution performance. Finally, the new index was validated by a simulation under the COMPASS navigation constellation.