This work presents a system-level thermal simulator, Phone-nomenon, to predict the thermal behavior of smartphone. First, we study the nonlinearity of internal and external heat transfer mechanisms and propose a compact thermal model. After that, we develop an iterative framework to handle the nonlinearity. Compared with a commercial tool, ANSYS Icepak, Phonenomenon can achieve two and three orders of magnitude speedup with 3.58% maximum error and 1.72°C difference for steady-state and transient-state simulations, respectively. Meanwhile, Phone-nomenon also fits the measured data of a built thermal test vehicle pretty well.