Abstract: Clearance Sealing is an essential sealing technology employed in downhole tools for heavy oil thermal recovery. Influenced by the alternating temperature conditions in the wellbore, both the dimensions of the clearance seal and the viscosity of the fluid change, thereby affecting the sealing performance. To address the internal leakage issue of clearance seals under complex temperature conditions, a prediction model for internal leakage of clearance seals considering temperature effects was developed based on the Annular Clearance Laminar Flow Leakage Model and the Steady-State Uniform Temperature Field Thermal Deformation Theory. The internal leakage variation of clearance seals with different initial piston diameters was investigated under high pressure of 34.5 MPa and a temperature range of 20～350 °C. Based on the internal leakage variation, a design chart for clearance seal dimensions was established. The results indicate that the internal leakage first increases and then decreases with the rise of temperature. The larger the initial piston diameter, the lower the temperature corresponding to the peak internal leakage and the faster the internal leakage decreases. Under wide-temperature-range working conditions, the clearance structure dimensions corresponding to the peak of the internal leakage curve should be used for design to meet the leakage requirements. When the clearance length is 100 mm and the piston diameter is 18 mm, an initial clearance of 0.009 mm can meet the requirement of internal leakage less than 20 mL/min. The established internal leakage prediction model for clearance seals can accurately predict the internal leakage of clearance seals under complex temperature conditions. The developed design chart for clearance seal dimensions can effectively guide the dimensional design of clearance seals to ensure the reliability of the sealing system.