In this 12-month randomized active-controlled clinical trial, we compare two post-treatment instructions for silver diamine fluoride (SDF) therapy in arresting dentine caries. The trial will include at least 254 kindergarten children with active dentine caries. The children will be randomized into two groups and receive a 38% SDF solution applied topically to their carious lesions. Children in Group A will rinse immediately, whereas those in Group B will refrain from rinsing, eating, and drinking for 30 min. One trained examiner will perform the dental examination at baseline and every six months. The primary outcome measurement will be the proportion of caries lesions that become arrested at the 12-month examination. Potential confounding factors and parents’ satisfaction with SDF therapy at baseline and after 12 months will be collected using parental questionnaires. This trial will provide evidence-based information for clinical practitioners to give post-treatment instructions for SDF therapy. This study is registered at ClinicalTrial.gov (USA) (registration number: NCT05655286).