Abstract: Clearance seal is an important sealing technology applied in downhole tools for heavy oil thermal recovery. Influenced by thermal cycling conditions downhole, the clearance seal dimensions and oil viscosity 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 established 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 a 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. It is found that the internal leakage first increases and then decreases with the increase of temperature. Moreover, a larger initial piston diameter means a lower temperature corresponding to the peak internal leakage and a faster decrease rate of the internal leakage. The research results show that under wide-temperature-range conditions, the clearance structure dimensions corresponding to the peak of the internal leakage curve should be used for design to ensure that the internal leakage meets the requirements. When the clearance length is 100 mm, and the piston diameter is 18 mm, an initial clearance design of 0.009 mm can meet the requirement that the internal leakage is less than 20 mL/min. The established prediction model for internal leakage of clearance seals can predict the internal leakage of clearance seals under complex temperature conditions. The established design chart for clearance seal dimensions can guide the dimensional design of clearance seals to ensure the reliability of the sealing system.