To assist Chinese high school students in improving their career readiness and tackling career decision-making difficulties, we designed a synchronous online career intervention based on the Cognitive Information Processing theory during the Covid-19 pandemic. The online career intervention consisted of a series of career courses to develop high school students’ knowledge and skills in career planning, career assessments for exploring their vocational interests and academic self-concept, and a database providing basic information about university majors. To evaluate the intervention’s effectiveness, 957 (...) 10th grade students were recruited in the study, 601 participants were randomly assigned to the experimental group, and 356 participants were randomly assigned to the control group. All participants completed a pre- and post-intervention assessment of their career maturity, vocational identity and career decision-making difficulties. Results indicated that the online intervention significantly increased high school students’ career readiness and reduced their career decision-making difficulties. The practical implications of this research for online career interventions directed at Chinese high school students are also discussed. (shrink)