In situ transformation and dissolution–crystallization mechanisms play a competing role in determining the characteristics of perovskite films that greatly affect the device performance of perovskite solar cells in the sequential two-step process. Herein, we develop a facile solution engineering to balance the transformation from PbI2 to CH3NH3PbI3 and dissolution–crystallization of CH3NH3PbI3 crystal growth, producing pure phase CH3NH3PbI3 crystals for high-efficient planar-structure solar cells. Low concentration of CH3NH3I in a mixed solvent of isopropanol/cyclohexane with low polarity is applied to suppress dissolution–crystallization (Ostwald ripening growth) of perovskite, while increases the transformation time from PbI2 to CH3NH3PbI3. Combination of porous PbI2 and temperature-assistance effectively promote the transformation from PbI2 to CH3NH3PbI3 and reduce the time of Ostwald ripening growth of perovskite. This solution engineering reconciles the complete PbI2 transformation and dissolution–crystallization of CH3NH3PbI3, resulting in a pure phase perovskite without any residual PbI2 in a short time. This strategy exemplified here can serve in the design and development of more sophisticated perovskites based on planar-structure applications without mesoporous TiO2 scaffold.