Disclosed is a structure for precision control of electrical impedance of signal transmission circuit board. A substrate forms thereon a plurality of first signal transmission lines, and a first covering insulation layer is formed on a first surface of the substrate to cover a surface of each first signal transmission lines and each spacing section formed between adjacent first signal transmission lines. Each first signal transmission lines can transmit a differential mode signal or a common mode signal. At least one first flattening insulation layer is formed between a surface of the first covering insulation layer and a first conductive shielding layer so that the first flattening insulation layer fills up the height difference between the surface of each first signal transmission line and the spacing section associated with each first signal transmission line to thereby ensure a consistent distance between the signal transmission lines and the conductive shielding layer for realizing precision control of electrical impedance of the signal transmission circuit board.