The invention provides a method for evaluating tightness of a high-temperature flange connection system. The method comprises the steps: analyzing positions, at which deflection angles of a flange generate; calculating a total deflection angle of the flange: calculating a deflection angle of the flange at the connection part between a cylinder section and a cone-shaped neck section by utilizing the connection part between the cylinder section and the cone-shaped neck section as the boundary, calculating a deflection angle of the flange at a connection part between the cone-shaped neck section and a flange plate by utilizing the connection part between the cone-shaped neck section and the flange plate as the boundary, calculating a rotation angle of the flange plate under the action of a bending moment by utilizing a disc rotation angle formula under the action of an external moment, calculating the sum of the three deflection angles and utilizing the sum as the total deflection angle of the flange; performing coordination analysis of axial displacement on a flange connection system, and solving a change rule of the stress of a spacer along with the time; evaluating the tightness of the flange; by integrating two aspects, namely the stress of the spacer and the total deflection angle of the flange, providing safe operation time of the flange in operating conditions. The invention provides a simple and effective quantitative evaluation method for the tightness of the high-temperature flange connection system, and offers reference basis to measure the safety of the high-temperature flange connection system.