The large bibliographic database Web of Science (WoS) includes an increasing number of early access (EA) records. The indexing method used for EA records in the database is different from the traditional method, but the comprehensiveness, accuracy and timeliness of the indexing are still unclear. Therefore, this article designs seven indicators for quantitative characterization based on five types of scenarios of EA indexing and then carried out empirical research with IEEE journals as a case. The results show that (1) in terms of comprehensiveness, not all EA journals and their EA records are indexed according to specific EA indexing methods (effective indexing) in WoS; only 26.3% of EA journals are indexed effectively and approximately 50% of EA records are indexed effectively in most EA journals. (2) In terms of accuracy, errors in DOI and journal websites have led to the abnormal phenomenon of unique EA records in WoS. Errors in the early access time and final publication time of EA records are common, highlighting the diversity problem of time indexing in WoS. (3) In terms of timeliness, more than 80.0% of the EA journals indexed effectively have indexing delays. The percentage of EA records with indexing delays ranges from 0.1% to 55.6%, and the weighted average index delay time for EA records ranges from 0.2 to 3.0 months. When constructing the WoS database and applying it for literature search, bibliometric analysis and research evaluation, all stakeholders should consider the potential impact of EA indexing and take corresponding measures.
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