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 the Trapezoidal and Backward Euler method.