The invention provides an improved fabrication process of a four-layer printed circuit board (PCB). Copper layers on the four-layer PCB are sequentially an L1 layer, an L2 layer, an L3 layer and an L4 layer. The fabrication process comprises the following steps of cutting to fabricate a substrate coated with the L2 and the L3; fabricating circuits of the L2 layer and the L3 layer; respectively covering and laminating the L1 layer and the L4 layer on the L2 layer and the L3 layer to form a symmetric four-layer lamination structure; drilling through holes in corresponding positions of the four-layer lamination structure by a mechanical drilling mode according to positions of through holes and blind holes which are designed in the circuits, wherein the through holes are also drilled in the positions of the blind holes; electroplating and processing the circuit drilled with the through holes; performing depth control drilling on the through holes in the positions of the blind holes according to the designed depths of the blind holes, and removing residual hole copper so that the hole copper of the through holes in the positions of the blind holes are only connected with a part of copper layers required to be connected in circuit design; performing resin hole plug on parts, of which residual hole copper is removed, of the through holes to form blind holes; fabricating circuits of the L1 layer and the L4 layer; and performing subsequent processes.