Since the first report in 2009, the certified power conversion efficiency (PCE) of organic-inorganic halide perovskite solar cells (PSCs) has rapidly risen to 23.3%, which successfully demonstrates the importance of film morphology and quality management, especially for the planar heterojunction architecture based on solution-processed method. In this paper, the morphology, structure, optical and electrical properties of perovskite films treated by two different multi-step thermal annealing methods have been studied and compared. For the conventional multi-step (CMS) annealing method, the surface morphologies and optical properties of films are significantly influenced by the annealing temperature due to the decomposition of CH3NH3PbI3. When the modified multi-step (MMS) method is adopted, the decomposition of CH3NH3PbI3 can be effectively suppressed. The best device treated by MMS method at 180 degrees C showed a short circuit current density of 20.85 mA/cm(2), an open circuit voltage of 0.96 V, a fill factor of 72.7%, and a PCE of 14.7%,. which is a 23.5% enhancement in PCE relative to the maximum 11.9% treated by CMS method at 160 degrees C. These findings suggest that the efficiency of PSCs can be further improved by optimizing the preparation process.