The invention discloses a spray coating mechanism. The spray coating mechanism comprises a left half ring, a right half ring and a holding rod. A fixing base is arranged between the left half ring andthe right half ring, the end of the left half ring and the end of the right half ring are hinged to the fixing base through hinged shafts, a permanent magnet is fixedly arranged at the top of the left half ring, and a cavity which is mutually attracted with the permanent magnet is formed in the corresponding position at the top of the right half ring, and when the cavity is mutually attracted with the permanent magnet, the left half ring and the right half ring are clamped to each other so as to form a ring. The holding rod is fixedly connected with the fixing base, a sliding groove in the axial direction is formed in the holding rod and matched with a slider, and the slider is hinged to one end of each one of two connecting rods. The spray coating mechanism has the advantages that the left half ring and the right half ring can be separated and clamped, a tree trunk can be conveniently sleeved between the left half ring and the right half ring, and the tree trunk is easily whitened thoroughly; the movement of the left and right half rings can be achieved through the movement of the slider, the operation is very simple and convenient, and the tree trunk whitening work efficiency isgreatly improved.