A relatively simple model is developed to simulate the widely tunable double-ring resonator lasers. In the simulation, the active region of the device is modeled using conventional time domain traveling method, because when the tuning range is large than 40 nm, the gain shape should be considered. The passive parts are first modeled by frequency domain method and then transformed into time domain via digital filter. The simulation result shows that the coupling coefficient has a significant effect on large signal modulation. With low coupling coefficient the laser can get narrow linewidth but poor modulation performance. Finally, the switching characteristics such as switching delay and multi-mode competition are also investigated. The result shows that the laser can switch the wavelength with high speed and large range.