Data on safety and immunity of heterologous booster (fourth dose) after three-dose priming with inactivated SARS-CoV-2 vaccine are limited in Chinese adults. Here, we evaluate the safety and immunogenicity of immunization with Ad5-nCoV in a randomized, double-blind, parallel-controlled phase 4 clinical trial in Zhejiang, China (NCT05373030). Participants aged 18-80 years (100 participants per group) who have administered three doses of inactivated SARS-CoV-2 vaccine ≥ 6-month earlier were enrolled and randomized at 1:1 into two groups, then administered intramuscular Ad5-nCoV or different inactivated SARS-CoV-2 vaccine (CoronaVac or Covilo) respectively. All observed adverse reactions were predictable and manageable. Ad5-nCoV elicited significantly higher RBD-specific IgG levels, with a GMC of 2924.0 on day 14 post-booster, 7.8-fold that of the inactivated vaccine. Pseudovirus-neutralizing antibodies to Omicron BA.4/5 showed a similar pattern, with GMT of 228.9 in the Ad5-nCoV group and 65.5 in the inactivated vaccine group. The Ad5-nCoV booster-maintained a high antibody levels on day 90, with seroconversion of 71.4%, while the inactivated vaccine group was 5.2%, closed to pre-booster levels. In summary, a fourth Ad5-nCoV vaccination following three-dose inactivated SARS-CoV-2 vaccination is immunogenic, tolerable, and more efficient than inactivated SARS-CoV-2 vaccine. Ad5-nCoV elicits a stronger humoral response against Omicron BA.4/5 and maintains antibody levels for longer than homologous boosting.