College students generally have good knowledge about COVID-19 and may facilitate COVID-19 vaccination in family. The purpose of this study is to understand college students' willingness to persuade their grandparents to initiate COVID-19 vaccination and the effect of their persuasion. A combined cross-sectional and experimental study will be conducted online. In the cross-sectional study (Phase I), eligible participants are college students who are aged ≥ 16 years and have at least one living grandparent aged ≥ 60 years who has/have not completed the COVID-19 vaccination. Participants self-complete Questionnaire A to collect information on the socio-demographics of themselves and their grandparents, their knowledge about older adults' COVID-19 vaccination, as well as Health Belief Model (HBM) and Theory of Planned Behavior (TPB) predictor variables. The primary outcome at Phase I is college students' willingness to persuade grandparents to receive COVID-19 vaccines. Those who are willing to persuade grandparents and participate in a follow-up survey will be invited to participate in a randomized controlled trial (Phase II). At Phase II, eligible participants are those who have at least one living grandparent aged ≥ 60 years who completed the COVID-19 initial vaccination series but has/have not received a booster dose. At the baseline, participants self-complete Questionnaire B to collect information on individual grandparents' COVID-19 vaccination status, attitude towards and intention to COVID-19 booster dose. Participants will then be randomly allocated 1:1 to either intervention arm (one-week smartphone-based health education on older adults' COVID-19 vaccination plus two weeks' waiting period) or control arm (three weeks' waiting period). At the end of week three, participants in both arms self-complete Questionnaire C to collect information on their grandparents' COVID-19 vaccination status. The primary outcome at Phase II is the uptake rate of COVID-19 booster dose among grandparents. Secondary outcomes include grandparents' attitude and intention to get a COVID-19 booster dose. No previous study had measured the effect of college students' persuasion on COVID-19 vaccination uptake in older adults. Findings from this study will provide evidence for innovative and potentially feasible interventions that further promote COVID-19 vaccination in older adults. Chinese Clinical Trial Registry: ChiCTR2200063240. Registered 2 September 2022.