Compact, pinhole-free and PbI2-free perovskite films, are desirable for high-performance perovskite solar cells (PSCs), especially if large columnar grains are obtained in which the adverse effects of grain boundaries will be minimized. However, the conventional solid-state reaction methods, originated from the two-step method, failed to grow columnar grains of CH3NH3PbI3 in a facile way. Here, we demonstrate a strategy for growing large columnar grains of CH3NH3PbI3, by less-crystallized nanoporous PbI2 (In-PbI2) film enhanced solid-state reaction method. We demonstrated columnar grains were obtainable only when In-PbI2 films were applied. Therefore, the replacement of compact PbI2 by In-PbI2 in the solid-sate reaction, leads to higher power conversion efficiency, better reproducibility, better stability and less hysteresis. Furthermore, by systematically investigating the effects of annealing temperature and duration, we found that an annealing temperature >= 120 degrees C was also critical for growing columnar grains. With the optimal process, a champion efficiency of 16.4% was obtained and the average efficiency reached 14.2%. Finally, the mechanism of growing columnar grains was investigated, in which a V-Pb"-assisted hooping model was proposed. This work reveals the origins of grain growth in the solid-state reaction method, which will contribute to preparing high quality perovskite films with much larger columnar grains. (C) 2017 Elsevier B.V. All rights reserved.