Current source models are the methods of choice for gate-level delay and SI calculation in Deep Sub Micron regime. To fully utilize the information provided by the current source models, numerical integration is often applied to solve stage-based transient simulation that calculates delay, slew, or noise bumps. However, this is computationally expensive. In this paper, we present a fast and robust algorithm for delay and signal integrity (SI) calculation using current source models. By applying diagonalization and Sherman-Morrison formula together with a one-step Newton-Raphson method, the transient simulation cost of a stage with a single driver can be reduced from O(kmn3) to O(kn) with a small runtime overhead, where k is the number of time step, m is the average number of Newton-Raphson steps, and n is the size of matrices of the Reduced Order Model(ROM) of the parasitic network. The proposed method works perfectly with the popular implicit integration methods such as Trapezoidal and Backward Euler.