Multi-GNSS networks, for example M-GEX and iGMAS, are set up and grow quickly with the development of new navigation systems and multi-GNSS receivers. It is unavoidable that GNSS signals are deteriorated by several error sources. Thus GNSS network operators, data users and receiver designers all require data quality checking (QC). However, few GNSS data QC software can process data of new navigation systems even modernized GPS signals. This paper introduces BQC, a multi-GNSS data QC toolkit which can process data of GPS/GLONASS/Galileo/Beidou recorded in RINEX version 2 or 3. Various QC outputs are written in QC report files as well as graphically displayed. Also in this paper, the cycle slip detection processing of BQC is emphasized as a key algorithm and compared with that of TEQC, the most widely-used GNSS data QC toolkit. The numerical results show that BQC reduces miss probability in cycle slip detection and repairs cycle slip effects on multipath sequences more reliable compared to TEQC.