The COVID-19 pandemic has exacerbated anxiety and related symptoms among the general population. In order to cope with the mental health burden, we developed an online brief modified mindfulness-based stress reduction (mMBSR) therapy. We performed a parallel-group randomized controlled trial to evaluate the efficacy of the mMBSR for adult anxiety with cognitive-behavioral therapy (CBT) as an active control. Participants were randomized to mMBSR, CBT or waitlist group. Those in the intervention arms performed each therapy for 6 sections in 3 weeks. Measurements were conducted at baseline, post-treatment and 6 months post-treatment by Generalized Anxiety Disorder-7, Patient Health Questionnaire-9, Patient Health Questionnaire-15, reverse scored Cohen Perceived Stress scale, Insomnia Severity Index, and Snaith-Hamilton Pleasure Scale. 150 participants with anxiety symptoms were randomized to mMBSR, CBT or waitlist group. Post intervention assessments showed that mMBSR improved the scores of all the six mental problem dimensions (anxiety, depression, somatization, stress, insomnia, and the experience of pleasure) significantly compared to the waitlist group. During 6-month post treatment assessment, the scores of all six mental problem dimensions in the mMBSR group still showed improvement compared to baseline and showed no significant difference with the CBT group. Our results provide positive evidence for the efficacy and feasibility of an online brief modified MBSR program to alleviate anxiety and related symptoms of individuals from the general population, and the therapeutic benefits of mMBSR persisted for up to six months. This low resource-consuming intervention could facilitate the challenges of supplying psychological health therapy to large scale of population.