This paper evaluates messages with long-term observation data for BeiDou Satellite Based Augmentation System (BDSBAS) single-frequency (SF) service. The navigation, observation and augmentation data are collected by a Septentrio PolaRx5S receiver equipped in the National Key Laboratory of CNS/ATM and NovAtel GNSS-702/850 antennas located on the roof of New Main Building at Beihang University in China. By counting the number of alert and anomaly messages, the basic data format, especially the update intervals, for various BDSBAS messages is tested in accordance with international standards. Anomalies are declared when abnormal message length, preamble, parity or other events happen in RINEX data stream. Based on the analysis, it is found that no alerts but several anomalies exist from August to November in 2021, excluding those days on account of handover operations and laboratory power outages. Preamble and parity anomalies are caused by receiver decoding fault detected by cross validation with data streams from three BSABAS GEO satellites and from other monitoring institutions. By verifying integrity parameters including user differential range error (UDRE) and grid ionospheric vertical error (GIVE) decoded from BDSBAS integrity messages, it is shown that these parameters could envelop residual errors induced by satellite and ionospheric grid model in most of epochs. Most of the 32 GPS satellites and 117 ionospheric grid points meet the integrity requirements of 99.9% probability even with abnormal update messages, nevertheless 11 satellites and 41 ionospheric grid points cannot accomplish this goal currently. This envelope problem for UDRE and GIVE during the selected 85 days may be caused by the evaluation algorithms and alternative methods. By calculating the system coverage at 95%, 99%, 99.9% and 100% availability for LPV-200 service in China and surrounding areas, it is proved that BDSBAS is available within about 94% of Chinese mainland with 95% availability. Messages with time-out interval anomalies have greater impacts on the availability than those with update interval anomalies. The accuracy of position errors at Beihang university is 1.29 meters in horizontal direction and 2.64 meters in vertical direction. Furthermore, the availability problem that might be caused by limited ionospheric grid points at high latitude areas for BDSBAS is under investigation in our team and not introduced in this paper. These results are significant for testing current BDSBAS SF service and future dual-frequency multi-constellation (DFMC) service. Particularly, they can support the certification work to achieve APV-I and CAT-I capabilities in China and the surrounding areas.