Abstract: Accurately calculating the ECD of wellbore is the foundation for drilling parameter design and safe efficient drilling. To enhance calculation accuracy, a comprehensive model for ECD calculation in slim-hole horizontal wells under high-temperature and high-pressure conditions is developed. This model is based on rheological test data of drilling fluids under high-temperature and high-pressure conditions, and utilizes a multivariate nonlinear regression to get drilling fluid density and rheological parameters. By coupling the drilling fluid density with the rheological parameter regression model and the wellbore heat transfer model, a comprehensive ECD calculation model was established for slim-hole horizontal wells under high-temperature and high-pressure conditions. When compared to actual PWD data, the average relative error of the proposed model is 0.72%. The research shows that temperature and pressure impact greatly on drilling fluid density and rheological parameters when calculating ECD for small-diameter wellbores under high-temperature and high-pressure conditions. During the drilling process, as drilling fluid circulation time increases, the temperature of the lower wellbore section decreases, resulting in increased drilling fluid density and viscosity coefficient, and subsequently leading to increased annular pressure loss and ECD. Temperature gradient and drilling fluid displacement influence the distribution of annular temperature, thereby affecting annular ECD. The higher the geothermal gradient, the higher the annular temperature and the lower the annular ECD. And the lager the drilling fluid displacement, the lower the annular temperature and the higher the annular ECD. Factors such as drilling pipe rotational speed, annular size, and connection size play important roles in influencing annular ECD. As drilling pipe rotational speed increases, annular pressure drops, subsequently raising annular ECD, but the increase rate gradually diminishes. The larger the connection sizes, the smaller the corresponding annular sizes and the larger the annular pressure loss, and thus the higher the annular ECD. The research results can provide a theoretical basis for safe and efficient drilling in slim-hole horizontal wells under high-temperature and high-pressure conditions.