Abstract: Based on the knowledge that the structural time-dependent reliability can be calculated by solving a single equivalent performance function for a certain period, an improved method is proposed for time-dependent reliability calculation. First, the reason why the performance function is nonlinear is discussed for the simple beam and column members. Second, a specific phenomenon is found that there are two possible load values to satisfy the nonlinear limit state equation. Then, an imaginary variable is introduced, which considers the influences of resistant factors, thus to reduce the number of variables in response surface equation. So the equation can be expressed compactly. Finally, a concrete arch under full-span dead load and half-span live load is analyzed according to the proposed method. It shows that the ultimate load curves are highly nonlinear and the method is accurate.