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Peking University: ChiCTR2100041942.Studies have shown that mobile cessation programs can encourage smokers to quit smoking and are affordable even in resource-poor settings, but have mostly focused on showing the framework of an intervention and reporting how many smokers reach abstinence (normally the three- or six-month sustained abstinence rate) [As even the most effective treatments help just one in four smokers quit in the long-term, and as mobile cessation may have a much lower quitting rate than pharmacotherapy, exclusively measuring the quitting rate is insufficient [Until recently, the psychological mechanism of promoting quitting behaviour using mobile health interventions have been explored by only a few studies [Individuals are usually advised to quit smoking abruptly on a designated “quit” day [Reducing smoking frequency can also be a pathway of a positive change for smokers, as studies support the value of lengthening the time between cigarettes [Studies have found that personalised and behaviour change theory-based interventions could be more effective than only providing broad, non-specific advice [We previously conducted a randomised controlled trial (RCT) to address the evidence gap and comprehensively evaluate the impact of mobile cessation interventions. We found that a behaviour change theory-based smoking cessation intervention through personalised text messages was more effective in reducing the biochemically verified six-month sustained abstinence rate than one based on non-personalised messages (6.9% vs 3.0% (adjusted odds ratio (OR) = 2.66; 95% confidence interval (CI) = 1.21-5.83)) [Here we aimed to evaluate the efficacy of a behaviour change theory-based smoking cessation intervention delivered through personalised text messages from the perspective of smokers who fail to reach abstinence. We focused on behaviour change, such as reducing smoking amount or frequency, of participants who continued smoking during the follow-up period. We also further sought to explore the psychological mechanism of positive change.