Abstract: A tangent angle circuit in a liquid crystal display (LCD) driving system comprises: a charging module integrated on a control board for receiving input of a direct current (DC) driving voltage and outputting a cut-in voltage to charge a plurality of scan line driving circuits; a plurality of discharging modules integrated on the scan line driving circuits respectively for controlling the corresponding scan line driving circuits to discharge; and a plurality of external regulating modules externally connected to the scan line driving circuits respectively and connected to the corresponding discharging modules to regulate the discharging modules so as to control a discharging rate of the scan line driving circuits. In the present invention, a discharge slope of the corresponding scan line driving circuit can be regulated; thereby, the tangent angle circuit is suitable for use with scan line driving circuits having different parasitic capacitors.