The invention belongs to the field of the low-alloy steel production technology and particularly relates to low-carbon and extra-thick TMCP type Q420qE bridge steel and a manufacturing method thereof. The method includes the following technology route of KR molten iron pretreatment, converter smelting, LF and RH refining, continuous casting, casting blank slow cooling, continuous casting blank heating, rolling, pre-straightening, controlled cooling, hot straightening, stacking for slow cooling and cutting finishing. According to the method, production of the low-carbon TMCP type Q420qE bridge steel with the thickness ranging from 60 mm to 80 mm is achieved, low-temperature impacts and the strength surplus are ensured, and the welding performance is improved. According to a produced steel plate, the yield strength ranges from 450 Mpa to 520 Mpa, the tensile strength ranges from 560 Mpa to 640 Mpa, the percentage elongation after fracture ranges from 21% to 26%, and the Charpy impact energy (in the transverse direction) can reach 240 J or above at the temperature of minus 40 DEG C.