The wet lead (II) iodide (PbI2) film-based two-step spin-coating procedure is employed to prepare good-performed planar heterojunction (PHJ) solar cell by simply controlling the remaining content of dimethyl sulfoxide (DMSO) solvent in PbI2 film. It was found that such wet PbI2 film was beneficial to produce pinhole-free, smooth, and highly crystalline perovskite film since the remaining DMSO could retard the crystallization of PbI2. By modulating the content of residual DMSO, the best performed device has efficiency as high as 21.20% with negligible photocurrent hysteresis. The average conversion efficiency has reached 20.15%, 14.42% enhanced comparing to annealing method. Our technique is simple but effective, and we expect it could be widely used especially for devices fabricated on flexible substrates.