BACKGROUND: While previous studies indicated that seeking online health information could reduce individuals' cancer information overload, the results are inconsistent and have remained unknown in China. This study focuses on cancer patients' family members to determine whether online health information seeking helps lessen cancer information overload and the processes underlying this association. OBJECTIVE: To examine the relationship between online health information seeking and cancer information overload through psychological empowerment and anxiety in the sequel, we carried out a quota sampling online survey in mainland China in 2023. We also looked at the underlying mechanism's moderated role in eHealth literacy. METHODS: We standardized all variables from 0 to 1 using a Min-max normalization and conducted Model 6 and Model 92 of Process Macro to examine the mediation and moderation effects. The final sample size was 628 cancer patients' family members. RESULTS: We found that online health information seeking negatively impacted cancer information overload through psychological empowerment and anxiety (  = -.007, CI: [-.013, -.002]). Specifically, online health information seeking was positively related to psychological empowerment (  = .201, CI: [.149, .252]), which eased family members' anxiety (  = -.271, CI: [-.420, -.122]) and eventually reduced cancer information overload (  = .120, CI: [.063, .177]). Moreover, we observed that while online health information seeking increased family members' anxiety (  = .126, CI: [.023, .228]), eHealth literacy served as a moderator to mitigate this association (  = -.668,  < .05). CONCLUSION: The findings can be used by healthcare workers, public health policymakers, and online health information providers to advise Chinese cancer patients' family members about the overwhelming amount of information they may encounter when seeking online health information.