INTRODUCTION: Children and young people (CYP) presenting with a mental health (MH) crisis are frequently admitted to general acute paediatric wards as a place of safety. Prior to the pandemic, a survey in England showed that CYP occupied 6% of general paediatric inpatient beds due to an MH crisis, and there have been longstanding concerns about the quality of care to support these patients in this setting. Mental Health Admissions to Paediatric Wards Study aims to generate a theory of change (ToC) model to improve the quality of care for CYP admitted to acute paediatric services after presenting in a MH crisis. METHODS AND ANALYSIS: We will undertake a national (England), sequential, mixed methods study to inform a ToC framework alongside a stakeholder group consisting of patients, families/carers and healthcare professionals (HCPs). Our study consists of four work packages (WP) undertaken over 30 months. WP1 is limited to using national routine administrative data to identify and characterise trends in MH admissions in acute paediatric wards in England between 2015- 2022. ETHICS AND DISSEMINATION: WP1 received ethical approval (Ref 23/NW/0192). We will publish the overall synthesis of data and the final ToC to improve care of CYP with MH crisis admitted to general acute paediatric settings. As coproducers of the ToC, we will work with our stakeholder group to ensure wide dissemination of findings. Potential impacts will be on service development, new models of care, training and workforce planning.