Uniform and dense perovskite films were realized by the one-step solution-processing method combined with toluene washing. The influence of the delay time applied for toluene washing on the film quality of CHNHPbI (MAPbI) was investigated in a comprehensive manner. The optimal delay time was experimentally observed at the critical point when the color of the film changes from transparent to hazy. A detailed X-ray diffraction study suggested that such a color change was caused by the emergence of the MAPbI crystal nucleus. This finding provides a convenient method to determine the optimal time accurately. With the optimal delay time, the most uniformly distributed MAPbI grains with the largest average grain size and the smoothest surface were obtained. Owing to the realization of homogeneous MAPbI films combined with full coverage of perovskite on the substrate achieved by toluene washing at the critical point, open-circuit voltage, short-circuit current, fill factor, and power conversion efficiency of 1.11 V, 18.24 mA/cm, 77.47, and 15.54% were obtained.