We present a simulation method to estimate the dark count rate (DCR), photon detection probability (PDP), and dark current of InGaAs/InP single-photon avalanche diodes (SPADs) from 225K to 300K. All simulations are performed completely in TCAD and match well with the measurement results of our novel selective area growth (SAG) based InGaAs/InP SPAD. An optimized simulation environment has the potential of estimating the device performance without costly fabrication iterations. Hence, it will accelerate the development of high-performance InGaAs/InP SPADs.