Fabrication of most-promising CH3NH3PbI3 based perovskite solar cell in ambient condition is excessively essential to industrialize this revolutionary development. In this research work, an efficient, facile and economical technique has been developed to fabricate CH3NH3PbI3 perovskite solar cell in ambient condition which is termed as dual-step thermal engineering technique. In this dual-step thermal engineering technique, the perovskite precursor solution has been spin coated over a mildly hot substrate which was heated at 60 degrees C for 10 min and followed by annealing at 80 degrees C for 30 min immediately after spin coating and compared with the perovskite film fabricated by conventional annealing in which the film was heated once at 120 degrees C for 60 min after spin coating. The comparative study shows that the newly developed dual-step thermal engineering technique is highly efficient and forms smooth, dense, well-crystallize, almost pinhole free perovskite solar cell at open air condition and exhibit remarkable enhanced efficiency over conventional annealing. This is because the precursor solution spontaneously started to develop a perovskite layer over the substrate during spin coating similarly as epitaxial-growth by utilizing the kinetic energy of the hot substrate and it serves as foundation layer for the high quality device.