The invention provides a preparation method of an inner layer of a thick copper thin circuit board of a printed circuit board. The technical scheme of the preparation method includes that plating grinding and chemical micro-etching are carried out in preliminary treatment, secondary printing ink is coated in coating, exposure is carried out with a semi-automatic exposure machine of a brand Chuanbao and a model E2100-7KMD, and an encrypted nozzle is adopted and compensation etching is carried out on the line width of a film. The preparation method has the advantages that the reject rate due to open circuits and gaps can be reduced to be 1.5%; the value of the line width is within +/- 10%; and exposure foreign matters and image distortion are reduced. The defects that the reject rate of an existing thin circuit board caused by open circuits and gaps is high, edges of circuits after etching are not regular, thicknesses of the circuits are not uniform and the range of line widths is large are overcome.