A compact NBTI model is presented by directly solving the reaction-diffusion (RD) equations in a simple way. The new model can handle arbitrary stress conditions without solving time-consuming equations and is hence very suitable for analog/mixed-signal NBTI simulations in SPICE-like environments. The model has been implemented in Cadence ADE with Verilog-A and also takes the stochastic effect of aging into account. The simulation speed has increased at least thousands times. The performance of the model is validated by both RD theoretical solutions as well as silicon results.