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Introduction This article outlines the research protocol for a multicentre, randomised, controlled study designed to evaluate the therapeutic effect of a modified olfactory training (MOT) based on bi‐directional nasal drug delivery system for patients with postinfectious olfactory dysfunction (PIOD), and to compare its efficacy with conventional olfactory training (COT).Methods and analysis This is a multicentre study in which patients will be recruited from several participating hospitals. Patients will be divided into three groups: COT group using COT device, MOT group using MOT device, Control group without any intervention other than follow-up. The olfactory training (OT) intervention will last for 12 months. The primary outcome will be the improvement in olfactory ability from baseline measurement to the end of intervention or control period, evaluated through the total Threshold, Discrimination, Identification (TDI) score of the Sniffin’ Stick test. Secondary outcomes will be changes in olfactory bulb volume and shape, olfactory-related brain area volume, olfactory and trigeminal nerve-related potentials, and subjective assessments.Ethics and dissemination This study protocol has been registered with ClinicalTrials.gov. The Peking University Third Hospital Medical Science Research Ethics Committee reviewed and approved this study protocol. The results will be published in BMJ Open.Trial registration number NCT06829706.