Introduction The efficacy of multitarget neuroprotective drug DL-3-n-butylphthalide (NBP) in improving cognitive function has been confirmed in patients with vascular cognitive impairment without dementia. However, its efficacy in patients with symptomatic predementia phase of Alzheimer's disease remains uncertain. This study aims to evaluate the efficacy and safety of NBP in improving cognitive function in patients with mild cognitive impairment (MCI) through a clinical randomised controlled trail.Methods and analysis This study is a 12-month, randomised, double-blind, placebo-controlled, multicentric trial, involving 270 patients with MCI. Subjects are randomly assigned to receive either NBP soft capsule (200 mg, three times per day) or placebo with an allocation ratio of 1:1. The efficacy and safety of NBP are assessed by comparing the results of neuropsychological, neuroimaging and laboratory tests between the two groups. The primary endpoint is the change in Alzheimer's Disease Assessment Scale-Cognitive Subscale after 12 months. All patients will be monitored for adverse events.Ethics and dissemination This study involving human participants has been reviewed and approved by Ethics Committee of Xuan Wu Hospital (No.2017058). The participants provide their written informed consent to participate in this study. Results will be published in peer-reviewed medical journals and disseminated to healthcare professionals at local and international conferences.Protocol version V 3.0, 3 September 2022.Trial registration number ChiCTR1800018362.