The annealing temperature (T_a) effect on CH₃NH₃PbI₃ perovskite solar cells (PSCs) was studied. By utilizing a two-step technique, the T_a dependences of the optical absorption, grain size, and crystallinity of a CH₃NH₃PbI₃ thin film have been revealed. It is found that the grain size of the CH₃NH₃PbI₃ film increases monotonically with T_a. Meanwhile, the decomposed PbI₂ emerges when T_a exceeds 120 °C and its content increases rapidly as T_a increases further. Consequently, the optical absorption of the CH₃NH₃PbI₃ film and the efficiency of PSCs reach their maximum at T_a = 120 °C simultaneously. The highest and average device performances of PSCs achieved via this method are 17.61% and 16.40%, respectively. These results confirm the key role played by temperature and provide a route to the performance-optimization of PSCs.