The invention provides a method for preparing a pretreated Low Dk type glass fiber fabric for forming a circuit substrate, wherein the method comprises that a pretreatment glue liquid having the Dk close to the Dk of the used Low Dk type glass fiber fabric at different temperatures and having small Df is used for pretreatment of the Low Dk type glass fiber fabric. The invention also provides an adhesive sheet and the circuit substrate prepared by the method. According to the preparation method of the circuit substrate, the Dks of the prepared circuit substrate have small difference in the warp direction and the weft direction, the problem of signal time delay can be effectively solved, the Df of the circuit substrate is relatively small, and the signal loss of the circuit substrate is also relatively small. At the same time, a cured, partially cured or uncured dried glue after a solvent of the pretreatment glue liquid is dried has dielectric properties at different temperatures close to those of the Low Dk type glass fiber fabric, so that the circuit substrate has quite small signal time delay at different temperatures.