Integrity is one of the most important indicators of Satellite-Based Augmentation System. According to “Global Positioning System Wide Area Augmentation System (WAAS) Performance Standard” released by the FAA and “Wide-Area Augmentation System Performance Analysis Report” released by NSTB quarterly, SBAS mainly evaluates the number of HMI events and minimum safety margin index of the monitoring station as major indicators of integrity performance. However, it is widespread protection level is far greater than position error of SBAS, it is difficult to accurately measure the envelope of the protection level for the positioning error simply by minimum safety margin index of the monitoring station. In view of this phenomenon, this paper proposes the concept of tightness, which can directly measure the degree of matching of the protection level to the position error throughout the service time, and at the same time, in the case of satisfying the protection level envelope position error, the range of degrees will be limited to the [0,1) range for easy observation. On this basis, this paper uses WAAS measured data for verification analysis, which proves that the tightness, minimum safety margin index of the monitoring station and the number of HMI events can help more accurately describe and evaluate the integrity.