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.