We present an accurate, easy-to-use large-signal SPICE circuit model for depletion-type silicon ring modulators (Si RMs). Our model includes both the electrical and optical characteristics of the Si RM and consists of circuit elements whose values change depending on modulation voltages. The accuracy of our model is confirmed by comparing the SPICE simulation results of 25 Gb/s non-return-to-zero (NRZ) modulation with the measurement. The model is used for performance optimization of monolithically integrated Si photonic NRZ and pulse-amplitude-modulation 4 transmitters in the standard SPICE circuit design environment.